Spoke to Apple support for about an hr about this and opened a ticket. Seems to have cleared up today.

Getting this more since the holidays. What's the problem? Is it me or Apple?

I have sporadically been getting the same on some of my apps. Since part of my team are business people without developer accounts, I push out builds to external testing so that they can download and see the work in progress. Spoke to support about this months ago and the junior support lady said they wanted the app to be further along before going external. I ask how do I let stakeholders see the work in progress. She was unsympathetic, so appealed and her boss overrode her. Now getting these "There was an error processing your request. Please try again later." messages randomly and wondering if it is related. Just need a way to show build members and marketing builds as the team works on the app and seems there sometimes it works and sometimes it doesn't. I don't get it.

Haven't tried it recently, but last year, I created a new separate account on my MBP and moved all of my work and mail over to it. Mail was a pain to move, but it seemed to solve a lot of problems at the time. Having too many email accounts in Mail seems to be a part of the problem, especially if they were Office365 emails. Disabling some of the emails in Preferences calmed things down, but I need them for work, so not sure what to do. Funneling several email accounts into one helped, but did not fix the problem.