I came across this error today while deploying some SSRS reports to a server using the SSDT BI component for Visual Studio.
The path of the item ‘/DataSources//Database’ is not valid. The full path must be less than 260 characters long; other
restrictions apply. If the report server is in native mode, the path must start with slash.
I made the mistake of googling just the error message from the VS Error List window. That did not help. However, upon closer inspection of the Output window, it became clear that it was not the data source itself that was causing the problem, but one of its dependencies: a dataset.
I double-clicked the dataset in question, clicked ‘OK’ on the Shared Dataset Properties window, rebuilt, and the problem disappeared. There must have been something in the dataset file that I had imported into my Report project that pointed to the wrong place for the data source.