(I'm not sure where the bug is, please reassing, thanks!)

Until some time ago (late 2022) the follwing worked without problem:

ssh -X user1@localhost
// start graphical app1, e.g. firefox

ssh -X user2@localhost
// start graphical app2, e.g. chromium

This no longer works. When I try to start app2, I always get:
Error: cannot open display: localhost:10.0

To start app2, I need to:
- close app1,
- type "exit" to close connection to user1@localhost
- type "exit" to close connection to user2@localhost
- ssh -X user2@localhost

And now I can start app2.

Until recently I could log into several machines (> 2) and run all
different apps there at the same time, no problem.