I was having the same problem. Dave Peterson's idea is what worked
for me. After I mapped a drive and directory to where the error
message was pointing to, I could open Excel / Tools / Options and get
to any of the tabs.

The secret is that you have to change: 1) the default file location on
the General Tab, and 2) the location for the AutoRecovery file location
on the Save Tab. The 2nd one is curious as I didn't even have
AutoRecover enabled...but the location could be seen greyed out.

After making these changes, I closed Excel and Disconnected the drive
that I had mapped. Reopened Excel and everything was back to "normal".