Recurrent authentication error for Email Pro with IMAP

Hi,
I have several E-mail Pro accounts, working fine on outlook, thunderbird, or mobile clients, but sometimes (every 2 or 3 weeks), all clients give me an authentication error (and doesn't receive/send mail of course), during or a few hours, or a few days.
During this issue, webmail works fine.

Anybody experiments the same issue??

I use IMAP, port 993 with SSL/TLS for ongoing server.

It's like server is working fine, but not on the IMAP port 993.
What do you think??

(OVH support says "all is fine", but no).

I forgot to say that after a few hours or days, all come back and works fine, without having made any change on the configuration.