I have browsed through many issues on this forum, and counted at least 4 other complaints that are the same as this. The Red triangle shows up when the connection to IMAP server is lost. That stops emails from being downloaded. All other compliants, ie mine and the 4, are after upgrading from V8 to V9.
One person suggested updating their certificate or something.
Please take this issue with the utmost urgency. I have v9 on two computers. Both connect to the same IMAP server. One just went Red triangle for the second time since upgrade. A common finding is after sending an email to yourself it starts working again, but this may not last.
Please take this seriously.
Regards
Grant