- is that server also the NIS server?

No it's the file server. Just wanted to remind that we experience the same errors with a local (non NIS) user.
 
- what NFS version are you using?
We do nightly DNF updates:

qemu-block-nfs-2.9.1-1.fc26.x86_64

sssd-nfs-idmap-1.15.3-5.fc26.x86_64

nfs-utils-2.1.1-5.rc5.fc26.x86_64

texlive-mfnfss-svn19410.0-33.fc26.2.noarch

libnfs-1.9.8-3.fc26.x86_64

libnfsidmap-0.27-1.fc26.x86_64

texlive-psnfss-svn33946.9.2a-33.fc26.2.noarch

 
- with what mount options?
/etc/fstab:

/dev/mapper/vg_sobolev-lv_root /                       ext4    defaults        1 1

UUID=c88d3708-a1c0-49c8-8ee8-xxxxxxx /boot                   ext4    defaults        1 2

UUID=F6D4-79B0          /boot/efi               vfat    umask=0077,shortname=winnt 0 2

/dev/mapper/vg_workstation-lv_swap swap                    swap    defaults        0 0

/dev/mapper/vg_workstation-lv_home /home       ext4 defaults       1 2

/dev/mapper/vg_workstation-lv_local /usr/local       ext4 defaults       1 2 
- are you using an automounter?

yes autofs
 
If so, does is make a difference if
you disable it and mount the required dir manually for a test?

I can do that for a test. Let me know what would be helpful.
 
- is /usr/local/bin identical on both servers?

Yes that is shared:

nis-server:/usr/local                    11G  5.7G  4.3G  57% /local/nis-server

 
- (out of couriosity: why are you still sticking to NIS in 2017?)

Legacy issue for ease of account creation each semester. I've been begging our overall IT to get Winbind authentication working with their Active Directory which we've tested and work but they have other priorities...at least for now.
 

> Info: Using ADSL link parameters 512/24/1/0.

So you are using arctica with ADSL. ADSL connections have been
modified twice after 3.5.99.7, so maybe you should try the newest
version, at least for improved speed.

Can you clarify? Update what package? X2Go on Mac is 4.1.1.1. On the server? We use this repo for nx-libs3.6:

rpm -q nx-libs

nx-libs-3.5.99.7-1.20170506git150cbc2.fc26.x86_64

name=Copr repo for nx-libs-3.6 owned by mikedep333

baseurl=https://copr-be.cloud.fedoraproject.org/results/mikedep333/nx-libs-3.6/fedora-$releasever-$basearch/

type=rpm-md

skip_if_unavailable=True

gpgcheck=1

gpgkey=https://copr-be.cloud.fedoraproject.org/results/mikedep333/nx-libs-3.6/pubkey.gpg

repo_gpgcheck=0

enabled=1


 
Is /tmp mounted via NFS?

Definitely not.