There are multiple ways to reproduce this bug. For example:
Let the server and x2goclient exchange an ecdsa key. Configure the server to only use rsa keys. Then attempt to connect with x2goclient. (This is what happened in the original bug report.) (This will happen when someone configures a server for FIPS compliance.) (Note that if both an rsa key and an ecdsa key are available, an ecdsa key will be exchanged.)
Let the server and x2goclient exchange an rsa key. Configure the server to only use ecdsa keys. Then attempt to connect with x2goclient. (This will produce the inverse error message.)
Let the server and x2goclient exchange an ecdsa key. Replace that key with an ecdsa key of a different length. Then attempt to connect with x2goclient.
What will not reproduce this bug is configuring a server to only use rsa keys, and then connecting with x2goclient for the 1st time. (Which will exchange an rsa key.)