You open the SharePoint Dashboard and you suddenly see this error:
An error occurred during an attempt to establish a connection to the external data source. The following connections failed to refresh. TfsOlapReport
Fair enough, something happened to the Excel Services. Does it? Actually not – and if you try opening that specific report you will see your local Excel refreshing the data and working as usual.
That specific error is a generic refresh error in our case. I tried back and forth on all the usual suspects – SSAS permissions, SSS token in the file, SharePoint settings, even firewall ports – but nothing changed.
Suddenly I noticed some reports were working (the Burndown one for example) while this (Active Bugs by Priority) didn’t. So what?
Looking at the Connection String I saw the Burndown report had MSOLAP.3 as a provider, while the broken report was using MSOLAP.5…
A quick double check on the SharePoint server (Manage Excel Services Application –> Trusted Data Providers) brought to the solution: MSOLAP.5 was not listed as a Trusted Data Provider.
Once I added MSOLAP.5 to the list everything worked as expected again and the reports were correctly showing.