I wanted to modify my Sharepoint Protection Group on DPM to backup Sharepoint 2013 so I can do item level restore. When expanding the server and Sharepoint farm I got presented with the following error:

So the first thing I did was check my VSS writers, you can do so as follows:

• VSSAdmin ListWriters

Everything seemed fine. Permissions were set on the SQL Server VSS writer service with a domain account. Restarted the service, no luck.

Only thing I found was that 1 of the SharePoint Databases was giving an error. It was the WSS_Usage DB. I removed this DB, created a new one and then checked. All the errors in event viewer were gone.

I now went back to DPM, edited my protection group and I was able to select my Sharepoint farm.

Hope it helps.