Sorry for my late reply.
So what if you put just
'guser1.' to the remote directory path (no lead slash)?
Sorry, I do not have an MVS server for testing, so I cannot give you a definitive answer.
Please attach a log file for the new tests in any case.
Then it looks fine the first time (just realised that). When I connect next time I get the error message again. And I can see that the Directories tab is changed to /'guser1.'.
Now I found the problem, I think. When "Remember last used directory" is enabled you update the Remote directory field and put a / in front.
So when I disabled that feature I can use the connection without error messages.
A very strange thing is that when I try to go up in the structure I always are send to / no matter where I am in the non UNIX structure.