Problem with iCloud email in OS Monterey

Installed OS Monterey this morning. Big mistake. It does not get iCloud email properly. I can see the heading but not the body of the email, even though it appears to be downloading it. No problem with gmail. Have spent many happy hours with Apple support, done various tricks like starting in safe mode, off and on again, even reinstalling Monterey again. Nada. The problem is still there. Any ideas?