Excel: Unable to process the request. Wait a few minutes and try performing this operation again.

Again, just a lovely error message that is a wash of information! :)

A user called trying to open an Excel File on SharePoint and was thrown back the error above. They only had Read access to the file. I tried on my side and it opened with no problems. This could be limited to people with Read access to the file.

Anyway, the following resolved the problem:

1. Go to Central Administration
2. Click on "Manage Web Applications"
3. Highlight the Web Application in question
4. Click on "Service Connections"
5. Untick "Excel Sevices Application", you may need to change the dropdown from "Default" to "Custom"
6. Click OK
7. Go through Steps 3 and 4 again
8. Select "Excel Services Application" and click ok
9. Restart Internet Explorer on the user's machine

Hey Presto. No explanation. :)