Tell us the account you want to use to open _vti_rpc.

1. It happens the first time I open a file when I try to close it. I have seen it with both Excel and Word. Once I close the file (which saves successfully even though I always X out of the auth prompt), I don't see the prompt for that file again. It does not happen with new files, only files that were saved to OneDrive before August.
2. I have Office 365 subscription and updates are downloaded and installed automatically; I'm fully updated.
3. Yes, I am signed into Office with the same account as I sign into OneDrive, which is also signed in on my computer.
4. Yes. On this computer, this is the only account that I have ever signed in with. I recently rebuilt the computer (after I quit the job that had a different Office account).

I am on Windows 7 x64, with 32-bit Office installed (the default).

Note that if this were a real auth prompt, it wouldn't list the server as _vti_rpc, which is not a server that would have my OneDrive documents on it.

Same here. Just noticed it so presumably last update.

same problem to me.

Same problem.

Only started in the last week or so for me.

Office 2016, W10 Pro

Auto-updates ON

Any response from MS yet?

I found one site where a Microsoft employee admitted the problem is due to the most recent update that was released (probably the august one).  I'll give the link here. It is German, but it is easy to read when translated. UP VOTE this so that people can see it !

https://answers.microsoft.com/de-de/msoffice/forum/msoffice\_word-mso\_win10-mso\_2016/einloggdialog-beim-speichern-von-vorhandenen/9323c310-fb51-42cb-b4ea-870686bfadf8?auth=1

Same issue, only in last week.