Outlook displays banner that says: Check your connection Open a browser to sign into ______.com and receive new emails

the 2 is probably the lan port number on your router. I get the number 4

Btw.: This fix only seems to work with Windows 10, Version 21H1 (exact version: 10.0.19043 Build 19043). It does not seem to work with Windows 10, version 20H2.

Same issue here. It occurs some days and disappears on other days - returning the next few days. After clicking the button in the banner it disappears for about 2-5 minutes, then it returns. I could not find a correlation to whatsoever.

Connecting from home network via ethernet to Microsoft Cloud Exchange. Completely unrestricted network access configured for my notebook in my FritzBox DSL router. Irrelevant whether in my company VPN or not.

My original issue  (i.e. the subject of this thread)  has so far not come back, but Outlook started acting squirrely in other ways last week. Notably, I'd type a line of text in an email and when I'd advance to the next line, the preceding text would disappear. Still there, just couldn't see it.  If you'd backspace it would reappear. When I started Outlook in Safe Mode, it would work normally.  I did no have any add-ins running. Three things I did and it's fixed now; not sure which of them helped, but sharing in case it helps with this too:

1. I tried repairing Microsoft Office:

Control Panel--> Programs and Features, right-click Microsoft Office, "Change". I did the quick and then the detailed repair. All that did was tell me I had to change from my 64 bit version of Office to a 32 bit version, because I had some runtime program for Microsoft Access (that one of my other programs needs to run) that was 32-bit.  I guess you can't mix and match. 

2. I created a new Outlook profile (hold down CTRL key while starting Outlook and you'll get a window popup to start in safe mode and also to add a profile.) 
3. I ran the system file checker:

https://support.microsoft.com/en-us/topic/use-the-system-file-checker-tool-to-repair-missing-or-corrupted-system-files-79aa86cb-ca52-166a-92a3-966e85d4094e

It did find errors and fixed them. 

After all this, I decided on a whim to try Outlook with my old profile, since it already had my email segregated into subfolder vs all in the Inbox, and lo and behold it worked normally. So far, so good. I'm not sure which of the above steps did the trick. 

Hope this helps!

Andy

Spoke too soon - it's back