Intermittent access and functionality problems with iCloud mail in browser

For the last several days, iCloud Mail hasn't been working in my browser. I can log into iCloud, and I can see all the messages in my Mail inbox and my archive folders, but 95% of the time I can't interact with anything and I can't view my drafts, sent mail, trash, or junk. I keep getting pop-up messages that say, for example, "Cannot load 'Drafts'. Messages cannot be loaded at this time. Try again later," or "Message cannot be marked as read at this time," or "Message cannot be flagged at this time." I can hit "reply" on a message, but then once I've written my email, it won't send the message or even save a draft. Then, the other 5% of the time, it's fine and works normally.

I've been using iCloud in my browser for years with no issues, so no idea what changed. All the other apps accessible from my browser in iCloud (Notes, Calendar, Reminders) seem to be fine. I have already tried updating my browser, clearing my cache, using a different browser (Safari instead of Chrome), and using my laptop in different places to make sure it's not an issue with the wifi connection in a specific place.

Mail still works fine on my phone, but it's a pain to do ALL my emailing from there, so any suggestions to fix this would be hugely appreciated.

[Re-Titled by Moderator]