I’m having a similar issue in Chrome tonight. The top of my page says " Oops, unable to connect.Try reloading?". The bottom of my page says “You have 30 unsaved changes, which will be saved when the server reconnects.”. Other than these two error messages, the page behaves fine.
Without doing anything on the page showing the error messages, I reopened the same workbook in another tab of my browser and it loaded with all of the most recent changes included so it looks like the original page ultimately synced with the server successfully, but just failed to clear the error notifications. Even after opening it successfully in the new tab, the original tab still had the error messages.
One possibly significant issue is that I worked on the page for several hours yesterday with no problem, then left the tab open and the PC idle for over 12 hours before I started to work on it again. I only started to notice these error messages after making some edits after leaving the tab idle for so long. Any chance that might be related? Not a big deal for me since no work was lost, but I thought I’d mention it anyway.