After the problem started, I have played with my acount settings in Modest. I have tried to re-create the account, but no luck. Currently I have the following settings:

Incoming:
server imap.gmail.com
Secure authentication ticked
Secure connection SSL (IMAP4S)
Port 993

Outgoing:
server smtp.gmail.com
Secure authentication: Login
Secure connection SSL
Port 465

I'm not using CSSU. I have updated my N900 regularly from the standard repositories since I got the phone in February 2012.

I have Modest version 3.4.8+0m5 and libtinymail 1.1.92+0m5