TB 115 - add security exception - hangs

I got used to TB portable asking to "confirm security exception" at location imap.mail.de:993 or imap.web.de:993. Regular installed Windows10 TB did never ask this nor does TB 115.
When I click "Get Certificate" this hangs for ever.
My E-Mail service provider suggests imap.mail.de:993 for TB. No special hint for TB portable.
I installed same TB 115.4.1 on Windows10. This does not ask for adding security exception AND works fine.
I installed a second fresh new TB portable 115, to prevent update problems. Same problem - hanging "get certificate".
Problem may be older. I use TB portable only when needed out of house.

I assume no installation or update problem. Other servers like t-online.de still work silent and fine - even on TB portable 115.

Any hint how to solve this problem, what to check?