Today I woke up and thought: "The weather could be worse." And it got worse.
Then I thought: "At least the servers are running, so it could be worse on a Monday morning." And it got worse. The Excel WebApps weren't working. With the best error message ever, thanks to Microsoft for clearing up the problem that might occured:
An error has occurred.
Please try again.
Actually. It's not okay. It's one of those error message that could mean anything. Maybe a planet in outer space just exploded and forced Excel WebApps to stop working.
So, my first idea was: Maybe the service is missing some rights or something. So I used a little PowerShell script, but that didn't help. I'm posting it so it may help you.
$web = Get-SPWebApplication
$web.GrantAccessToProcessIdentity("DomainName\ServiceAccount")
So I tried to search for this error, but nothing helped. I checked the App Settings, check if any error occurred.
In the end one simple thing did the trick: Restarting the service fixed the problem.
But why did this help?
Well, we deleted files from the TEMP directory... and there are all those Excel WebApp Temp files, which are needed to display. These files will get initialized when you restart the service. Problem solved.
*Update 7/16/12*
In my case I restarted the Excel Web Services.
Thanks to Taras Kun for his tip:
"I got it fixed simply by restarting Microsoft SharePoint Foundation Web Application service In Central Administration > Application Management > Services on Server."