KMail crashes when opening settings, also can't send mails

Until just recently, KMail worked perfectly fine. I had one Mail Account, sending and receiving worked just fine. Now I added a second Mail account and suddenly I can’t send any emails. I always get a “server authentication failed” error.
So I wanted to remove the second account and tried to open Settings->Configure KMail but KMail immediately crashes.

What’s the issue here?

Open it from terminal and share the errors you get, or look for the crash in the log file.

Where do I find the crash logs? I couldn’t find anything online.
When starting kmail from the terminal the CLI process exits after the program starts and I’m left with