IMAP email disappeared (possibly after upgrading to iPhone 13/iOS 15.0)

My work email has disappeared from all my devices!

I'm in my office on Monday morning, open mail on my iMac and my inbox is entirely empty. I still have Sent messages and Drafts, but fours years worth of incoming messages have vanished. Checked on my new iPhone, and the same thing; nothing in my work email account. I checked the webmail interface in my browser; no incoming email at all. Nothing deleted or archived, just gone.

The last time that I know I accessed my work email was from my laptop at home on Thursday. I was out of the office Friday, which is also when I got a new iPhone 13 mini. Switched everything from my old phone Friday afternoon, everything seemed to go fine, didn't think to check my work email at the time.

I have other email accounts that are fine (gmail and my personal mail on a POP account) on my phone. I guess it could be coincidence, but it looks very much like my work IMAP mail disappearing coincided exactly with my setting up and copying everything over to my new phone.

Anyone else seeing mail problems migrating/upgrading to new iPhone/iOS 15?