Odd box showing up in this excel file on every worksheet

[right2rina]

I cannot see to get rid of this box that just showed up randomly.

I have even opened a new sheet and it just shows up.  I can't even "click it to delete it"

Any ideas?

[#2]

This doesn't look like anything Excel would generate, more like a programming error from another application. (Some pop-up?) Or perhaps from an Excel plug-in? (Some unhappy dialogue box from PDF generation etc?)

[#3]

Looks like office account login box. Go to File | Account and resolve account error with Fix button, if present. If there's no fix option, sign out of the account then sign in again.