mailhandler checks for emails but does not load them into nodes - Security problem

Up until yesterday it worked perfectly but suddenly I get the following messages in my log:

1)
Mailbox xxx was checked and found 1 messegage.

2)
IMAP error in close_mailbox: SECURITY PROBLEM: insecure server advertised AUTH=PLAIN

3)
Notice: Unknown: SECURITY PROBLEM: insecure server advertised AUTH=PLAIN (errflg=1) in main()

How can I fix this? Thanks a lot for your help.