Yes, I have exactly the same issue. Has been a problem for months.
Same goes for loading 'messages' page which is just blank. Its all effing annoying.
Safari Version 15.3 (17612.4.9.1.8)
macOS Monterey ver12.2.1
Yes, I've cleared cache and restarted. Works for an hour or two, maybe a day then back to being a problem.