connection closed every ten minutes

[Marco Fioretti]

Greetings,

I use mutt on Ubuntu to access my IMAP mailboxes, on my Centos email
server that runs dovecot. Everything has worked without problems for
years. About one week ago, the connection between mutt and dovecot
became unstable.

Before, I could leave mutt connected for days in a row, no problem.
Now, everything still works fine, except... I get every ten minutes I
get "connection timed out" in Mutt's status line, and hundreds of
messages like

Apr 12 16:12:49 SERVERNAME dovecot: imap(ACCOUNTNAME): Logged out in=164 out=757

what puzzles me is that I did not touch anything both on my server and
on my desktop, except an "apt-get update" some days before this
started.

But cannot see how it would be related anyway, nor have I found
anything online like this.

Any help to understand what happened and fix it is very welcome.

Marco