K-9 mail setup readStringUntil()

Hi there,
On Android 11, I could setup one mail account without problems.
Then, while setting up a 2nd account I got this error:
Network error
Details:
readStringUntil(): end of stream reached. Read: “” while waiting for ’ '.

If I try to check the working account, I get the exact same error. What could change in between ?

I am running my own IMAP/SMTP servers, this behaviour happens while on the local network as well from outside.
I have no idea where to start to investigate. Seems so strange this behaviour suddenly shows up…

Anyone else got he same problem ?

Uh, oh…
Waited a few minutes and like magic it works again.
I love magicians inside a circus but am a tad afraid of them inside my devices…
Then again same exact problem. Any idea anyone ?
It might be on my server side, but, well good old exim is working quite well even if a huge thing…
Thanks for any clue given !

One more update.
It seems the problem lies with startTLS. If using SSL/TLS everything works fine.
But… my server actually uses startTLS for IMAP. Intensive head scratching on the range…
Will update when/if I find the real culprit, if not me.
Cheers !