The usual cause of an "Instrument Not Found" would be a messed-up or partial installation, and the cure would be to re-install the Hi-Q sounds (using the Dec. 2014 Hi-Q sounds installer that was included with the update), as well as the BB 2015 latest patch.

Here's a test that you could do to see if that is the case....

For example, lets test the “Classic Bass” sound.
You should have these 2 files, with these 2 dates: (assuming BB is installed to c:\bb)
1. c:\bb\DX Settings\Hi-Q 034 01 Bass, Electric, 61 Classic SampleTank.tgs dated 6/26/2014
2. C:\Program Files (x86)\IK Multimedia\SampleTank 2.5\Instruments\Band-in-a-Box\Bass\61 Classic Bass PG.stip dated 6/26/2014

If your dates are different, and you have BB 2015, then you should update the installation as follows:
To fix this problem (assuming version 2015)

1. Install the latest BB 2015 patch.
2. Install the latest 2015 HiQ instrument installer. (should be Dec 2014 or later) (e.g. hi-q_instruments_sampletank_01-02.exe)

Have Fun!
Peter Gannon
PG Music Inc.