By chance, I learned of the --libssh-debug and --libssh-packetlog options. So here's the output of a session that crashes. Any hints as how to pursue this further?
C:\Program Files (x86)\x2goclient>x2goclient.debug.exe --debug --libssh-debug --libssh-packetlog x2go-INFO-1> "Starting X2Go Client 4.1.2.2..." x2go-INFO-4> "Translator: :/i18n/x2goclient_de_de found." x2go-INFO-4> "Übersetzung: :/i18n/qt_de_de gefunden." x2go-DEBUG-../src/onmainwindow.cpp:10779> Getting X.Org Server settings. x2go-INFO-3> "X2GoClient wurde gestartet." x2go-DEBUG-../src/onmainwindow.cpp:575> "$HOME=C:/Users/ef" x2go-DEBUG-../src/onmainwindow.cpp:2266> Reading 1 sessions from config file. x2go-DEBUG-../src/sessionbutton.cpp:361> Creating QPixmap with session icon: ":/img/icons/128x128/x2gosession.png". x2go-DEBUG-../src/onmainwindow.cpp:13290> libssh not initialized yet. Initializing. x2go-DEBUG-../src/pulsemanager.cpp:369> pulseaudio --version returned:"pulseaudio 13.0 "
x2go-DEBUG-../src/onmainwindow.cpp:10682> Starting helper servers for Windows ... x2go-DEBUG-../src/onmainwindow.cpp:10849> "/cygdrive/C/Users/ef/.x2go/var" cygwin var path x2go-DEBUG-../src/onmainwindow.cpp:10937> "C:/Users/ef/.x2go/etc/sshd_config created." x2go-DEBUG-../src/onmainwindow.cpp:10344> Resolved localhost. x2go-DEBUG-../src/onmainwindow.cpp:10366> Port is free: 7022 x2go-DEBUG-../src/onmainwindow.cpp:11288> Logging cygwin sshd to: "C:/Users/ef/.x2go/sshLogs/Hp2320.log" x2go-DEBUG-../src/onmainwindow.cpp:11302> Creating desktop: x2go_ef x2go-DEBUG-../src/pulsemanager.cpp:227> pulse started with arguments ("--exit-idle-time=-1", "-n", "-F", "C:\Users\ef\.x2go\pulse\config.pa", "-p", "C:\Program Files (x86)\x2goclient\pulse\lib\pulse-13.0\modules", "--log-level=debug", "--verbose", "--log-target=file:C:\Users\ef\.x2go\pulse\pulse.log") - waiting for it to finx2go-DEBUG-../src/onmainwindow.cpp:10344> Resolved localhost. ish... x2go-DEBUG-../src/onmainwindow.cpp:10372> Port already in use: 7022 x2go-DEBUG-../src/onmainwindow.cpp:10344> Resolved localhost. x2go-DEBUG-../src/onmainwindow.cpp:10372> Port already in use: 7022 x2go-DEBUG-../src/onmainwindow.cpp:11399> User mode OpenSSH server started successfully. x2go-DEBUG-../src/onmainwindow.cpp:2752> Creating QPixmap with session icon: '":/img/icons/128x128/x2gosession.png"'. x2go-INFO-8> "Verbindung mit Server wird gestartet: aegidienberg:22" x2go-DEBUG-../src/onmainwindow.cpp:2853> Starting new ssh connection to server:"aegidienberg":"22" krbLogin: false x2go-DEBUG-../src/sshmasterconnection.cpp:168> SshMasterConnection, host "aegidienberg"; port 22; user "mtuser"; useproxy true; proxyserver "login.math.uni-bonn.de"; proxyport 22 x2go-DEBUG-../src/sshmasterconnection.cpp:248> Starting SSH connection without Kerberos authentication. x2go-DEBUG-../src/sshmasterconnection.cpp:250> SshMasterConnection, instance SshMasterConnection(0x3a10da8) created. x2go-DEBUG-../src/sshmasterconnection.cpp:495> SshMasterConnection, instance SshMasterConnection(0x3a10da8) entering thread. x2go-DEBUG-../src/sshmasterconnection.cpp:498> proxyserver: "login.math.uni-bonn.de"; proxyport: 22; proxylogin: "" x2go-DEBUG-../src/sshmasterconnection.cpp:168> SshMasterConnection, host "login.math.uni-bonn.de"; port 22; user ""; useproxy false; proxyserver ""; proxyport 0 x2go-DEBUG-../src/sshmasterconnection.cpp:224> Temporary session user name after config file parse: x2go-DEBUG-../src/sshmasterconnection.cpp:248> Starting SSH connection without Kerberos authentication. x2go-DEBUG-../src/sshmasterconnection.cpp:250> SshMasterConnection, instance SshMasterConnection(0x3a112e8) created. x2go-DEBUG-../src/sshmasterconnection.cpp:495> SshMasterConnection, instance SshMasterConnection(0x3a112e8) entering thread. x2go-DEBUG-../src/sshmasterconnection.cpp:575> Setting SSH directory to C:/Users/ef/ssh x2go-DEBUG-../src/sshmasterconnection.cpp:797> Session port before config file parse: 22 x2go-DEBUG-../src/sshmasterconnection.cpp:807> Session port after config file parse: 22 [2020/05/25 18:47:02.347704, 2] ssh_connect: libssh 0.9.3 (c) 2003-2019 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_pthread [2020/05/25 18:47:02.401163, 2] ssh_socket_connect: Nonblocking connection socket: 2228 [2020/05/25 18:47:02.401163, 2] ssh_connect: Socket connecting, now waiting for the callbacks to work [2020/05/25 18:47:02.401163, 3] ssh_connect: Actual timeout : 60000 [2020/05/25 18:47:02.438119, 3] ssh_socket_pollcallback: Received POLLOUT in connecting state [2020/05/25 18:47:02.438119, 1] socket_callback_connected: Socket connection callback: 1 (0) [2020/05/25 18:47:02.438119, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket [2020/05/25 18:47:02.491386, 3] callback_receive_banner: Received banner: SSH-2.0-OpenSSH_5.9 NetBSD_Secure_Shell-20110907-hpn13v11-lpk [2020/05/25 18:47:02.491386, 2] ssh_client_connection_callback: SSH server banner: SSH-2.0-OpenSSH_5.9 NetBSD_Secure_Shell-20110907-hpn13v11-lpk [2020/05/25 18:47:02.491386, 2] ssh_analyze_banner: Analyzing banner: SSH-2.0-OpenSSH_5.9 NetBSD_Secure_Shell-20110907-hpn13v11-lpk [2020/05/25 18:47:02.491386, 2] ssh_analyze_banner: We are talking to an OpenSSH client version: 5.9 (50900) [2020/05/25 18:47:03.714230, 3] ssh_client_select_hostkeys: Order of wanted host keys: "ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss" [2020/05/25 18:47:03.803831, 1] ssh_known_hosts_read_entries: Failed to open the known_hosts file '/etc/ssh/ssh_known_hosts': No such file or directory [2020/05/25 18:47:03.803831, 3] ssh_client_select_hostkeys: Algorithms found in known_hosts files: "ecdsa-sha2-nistp521" [2020/05/25 18:47:03.803831, 3] ssh_client_select_hostkeys: Changing host key method to "ecdsa-sha2-nistp521,ssh-ed25519,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,rsa-sha2-512,rsa-sha2-256,ssh-rsa,ssh-dss" [2020/05/25 18:47:03.803831, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket [2020/05/25 18:47:03.803831, 3] packet_send2: packet: wrote [type=20, len=940, padding_size=4, comp=935, payload=935] [2020/05/25 18:47:03.803831, 3] ssh_send_kex: SSH_MSG_KEXINIT sent [2020/05/25 18:47:03.803831, 3] ssh_packet_socket_callback: packet: read type 20 [len=812,padding=5,comp=806,payload=806] [2020/05/25 18:47:03.803831, 3] ssh_packet_process: Dispatching handler for packet type 20 [2020/05/25 18:47:03.822580, 2] ssh_kex_select_methods: Negotiated ecdh-sha2-nistp256,ecdsa-sha2-nistp521,aes256-ctr,aes256-ctr,hmac-sha1,hmac-sha1,none,none,, [2020/05/25 18:47:03.822580, 3] packet_send2: packet: wrote [type=30, len=76, padding_size=5, comp=70, payload=70] [2020/05/25 18:47:03.822580, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket [2020/05/25 18:47:03.875547, 3] ssh_packet_socket_callback: packet: read type 31 [len=428,padding=11,comp=416,payload=416] [2020/05/25 18:47:03.875547, 3] ssh_packet_process: Dispatching handler for packet type 31 [2020/05/25 18:47:03.875547, 3] ssh_socket_unbuffered_write: Enabling POLLOUT for socket [2020/05/25 18:47:03.875547, 3] packet_send2: packet: wrote [type=21, len=12, padding_size=10, comp=1, payload=1] [2020/05/25 18:47:03.875547, 3] crypt_set_algorithms2: Set output algorithm to aes256-ctr [2020/05/25 18:47:03.875547, 3] crypt_set_algorithms2: Set HMAC output algorithm to hmac-sha1 [2020/05/25 18:47:03.875547, 3] crypt_set_algorithms2: Set input algorithm to aes256-ctr [2020/05/25 18:47:03.875547, 3] crypt_set_algorithms2: Set HMAC input algorithm to hmac-sha1 [2020/05/25 18:47:03.875547, 2] ssh_init_rekey_state: Set rekey after 4294967296 blocks [2020/05/25 18:47:03.875547, 2] ssh_init_rekey_state: Set rekey after 4294967296 blocks [2020/05/25 18:47:03.875547, 2] ssh_packet_client_ecdh_reply: SSH_MSG_NEWKEYS sent [2020/05/25 18:47:03.875547, 3] ssh_packet_socket_callback: Processing 16 bytes left in socket buffer [2020/05/25 18:47:03.875547, 3] ssh_packet_socket_callback: packet: read type 21 [len=12,padding=10,comp=1,payload=1] [2020/05/25 18:47:03.875547, 3] ssh_packet_process: Dispatching handler for packet type 21 [2020/05/25 18:47:03.875547, 2] ssh_packet_newkeys: Received SSH_MSG_NEWKEY