File ID 5 of database ID 8 cannot be shrunk as it is either being shrunk by another process or is empty

"File ID 5 of database ID 8 cannot be shrunk as it is either being shrunk by another process or is empty"

 

One more thing to try that worked for me when I received this error message.

I just increased the data file by a few MB and then tryed to do the Shrink again. It worked fine.

It seems like adding a few MB to the data file resets an internal counter or switch that tells it it's not in the middle of a shrink now.