Session invalid or expired

[EPinci]

Hello there,

I'm experiencing a weird issue with roundcube 7.2.
My setup includes roundcube, mysql and an IMAP server all on the same box.
I can use fine roundcube logging against my IMAP server. So far, so good.

Then I decided to try to use roundcube also against my ISP's IMAP server.
I can successfully login and then, for a brief moment, I can see an updated list of my mailbox's email.
At this point roundcube throws me out with a "Your session is invalid or expired".
In the erro's logfile I can see a "login failed for MD5-DIGEST" but the email list I could see is updated so an initial login must have gone good.
I then enabled IMAP session logging and that log file shows only successful logins.

Can anybody help me understand what is going on?

Thank you.