I'm seeing that too, now that I look for it.
I suspect that it's likely to do with the reported
Google Cloud Outage.
I'm still getting mail notifications on my cell, but Thunderbird also fails. That suggests that Gmail IMAP4 is down.
ADDED:
Google's own status board is showing many services as down.
UPDATE: Gmail IMAP4 is working for me again. It may be fixed, or it may be transiently broken.
UP-UPDATE: From
status page:
"6/2/19, 6:30 PM - The problem with Gmail should be resolved. We apologize for the inconvenience and thank you for your patience and continued support. Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better."