popper stops checking email

popper starts and checks email as it should when i log in. but then it randomly stops checking email.

here's a typical log:
Checking email accounts at: Wed Aug 17 11:23:14 2011
Checking email accounts at: Wed Aug 17 11:24:14 2011
Checking email accounts at: Wed Aug 17 11:25:14 2011
Checking email accounts at: Wed Aug 17 11:26:14 2011
Warning: Cannot connect to IMAP account: imap.***. Next try in 30 seconds.
Checking email accounts at: Wed Aug 17 11:27:14 2011
Checking email accounts at: Wed Aug 17 11:28:14 2011
Checking email accounts at: Wed Aug 17 11:29:48 2011
Checking email accounts at: Wed Aug 17 11:30:48 2011
Checking email accounts at: Wed Aug 17 11:32:22 2011
Checking email accounts at: Wed Aug 17 11:33:22 2011
Checking email accounts at: Wed Aug 17 11:34:22 2011
Checking email accounts at: Wed Aug 17 11:35:22 2011
Checking email accounts at: Wed Aug 17 11:36:22 2011
Checking email accounts at: Wed Aug 17 11:37:22 2011
Checking email accounts at: Wed Aug 17 11:38:22 2011
Checking email accounts at: Wed Aug 17 11:39:22 2011
Checking email accounts at: Wed Aug 17 11:40:22 2011
Checking email accounts at: Wed Aug 17 11:41:22 2011
Checking email accounts at: Wed Aug 17 11:42:22 2011
Checking email accounts at: Wed Aug 17 11:43:22 2011
Checking email accounts at: Wed Aug 17 11:44:22 2011
Checking email accounts at: Wed Aug 17 11:45:22 2011
Checking email accounts at: Wed Aug 17 11:46:22 2011
Checking email accounts at: Wed Aug 17 11:47:56 2011
Checking email accounts at: Wed Aug 17 11:48:56 2011
Checking email accounts at: Wed Aug 17 11:49:56 2011
Checking email accounts at: Wed Aug 17 11:50:56 2011
Checking email accounts at: Wed Aug 17 11:51:56 2011
Checking email accounts at: Wed Aug 17 11:52:56 2011
Checking email accounts at: Wed Aug 17 11:53:56 2011
Warning: Cannot connect to IMAP account: imap.***. Next try in 30 seconds.
Error: Cannot connect to IMAP account: imap.***. Next try in 1 minutes.

and then nothing happens. new mails are not reported anymore.

this is on ubnutu 11-04 natty; popper 0.29.2-0ubuntu1 .

i checked that popper is still running after this happened:
$ cat popper.pid
30221
$ ps 30221
  PID TTY STAT TIME COMMAND
30221 pts/0 SLl 0:00 python popper.py autostarted