Hey PJB07! I’d still try the steps above, did you test it while in safe mode? Or just boot to safe mode then restart? Also can you see if it happens in a new admin account?
I know the one who posted this issue dima61 had removed all the internet accounts as a test before posting. Although that didn’t work for them, it’s worth a try. I’d either remove all, restart and test, then add one at a time in intervals making sure to test a while after adding each one, as it could be a single account causing troubles. I’ve fixed this myself by doing that, removing all accounts, I then copied then deleted the contents of ~/Library/Accounts, restarted, re added the accounts and all was good, this was on Mojave though. Also I knew for a fact it worked fine in another user. If it did not, I’d probably consider reinstalling the OS. But I don’t think there’s a single answer unfortunately for every case, unless it’s truly a bug that has a workaround or requires another update to fix.
But yes, knowing the results of those two tests will help (safe boot and a new admin account), as far as what direction to go. If it works fine in safe boot, I’d consider backing up, and removing specific files in the /Library and possibly ~/Library, let me know how it’s goes!