Outlook Classic failing to download entire O365 mailbox
I have an Office 365 E3 account that stopped downloading properly some time last week on 2/3 of my computers in Outlook Classic. They both constantly show the "Updating mailbox" notification at the bottom of Outlook, and if I compare between the 1 working computer and the ones that aren't, the ones that aren't working frequently get desynced when showing the number of unread emails in folders other than the current folder.
The mailboxes are currently 21 GB when viewed from the web & from the computer that's working correctly, and both computers have other accounts in Outlook working properly including a 23 GB mailbox, and they both have plenty of hard drive space, so I don't think the size is the issue. They're also set to download emails from all time in Outlook, although the mailbox itself has a 1 yr retention policy, so I don't think there's anything older than a year in it anyways.
One of the problem computers is on Microsoft Outlook for Microsoft 365 MSO 64-bit Version 2604, the other is on 2605. I've already tried repairing Outlook, clearing the cache, and creating a new profile. I did notice that after creating a new profile & redownloading, both of the problem computers stopped downloading at the exact same point (15,082,824 KB). I also tried disabling add-ins & starting Outlook in safe mode, but that made no difference. Any help with getting this fixed would be a appreciated.