I believe the original EB5 expansion pack was a DimPro pack. When RPro rolled out, several packages which I already owned showed up in C3. I did not install the "Gibson EB5 TCTN TCTB Bass (Win)" package because I already had installed it in DPro. Is this the one you are trying to use?
It sounds like the type of error one receives trying to use flac files in Rapture. Rapture Pro supports flac.