Posting because I see that people have been having this problem for years with no help from the QBO team.

I was having the same issue: on one specific account on the Bank Transactions page, I got the message: "Something’s not quite right. Sorry, an unexpected error has occurred. Please try again." QBO would show me there were two uncategorized transactions but wouldn't show what they were. I tried multiple browsers and devices, and even removed the account and re-added it. The problem persisted.

Solution: The transactions did show up on the mobile app (iOS). I tried to just "confirm" them as they are standard monthly recurring transactions, and the older of the two said "error" when I tried to confirm it. This led me to suspect that this specific transaction was the root of the problem, so I simply deleted it. Then I went back to the desktop and everything was back to normal. Subsequent new transactions are showing up correctly.

I don't have the technical knowledge to explain why a monthly recurring transaction that hasn't changed in three years is causing QBO problems all of a sudden, but in my case at least, this error was caused by one single transaction. We'll see if next month's baffles QBO again.