Customer Service Email Accounts with IMAPS

Hello

on one site we have an strange issue with IMAPS. It is a Lotus Notes server with latest version and we use a selfsigned certificate. When we switch to IMAPS, then quite often Service is not able to load the emails. It looks, that the oldest email is blocking all the others. Some times a solution is, to delete the oldest email or move it to a different folder.

But when we switch back to IMAP protocoll, then this issue does not happen. 

All needed ports are open and the certificate works. 

Does have anyone know similar issues and can give me a hint, what to do.

Thank you!