I am experiencing this same problem.

I use the Hi-Q instruments a lot - typically by loading a SuperMidi Piano track or Strings track (it then asks if you want to use the Hi-Q instrument).

I get the "part 3 : instrument not found message" on the piano track. If I go to the plugins tab it has Sampletank displayed. Bring that up and there is nothing selected - nothing in the channels. The Band In A Box folder IS listed on the right side.
And all the instruments are there. I can load the piano in channel 3. It will then play.

Select the strings and get "part 7 : instrument not found message". Go through the same thing - and make sure the Strings go into channel 7 - and they now both play.

Save the song, reload and get 2 errors:
part 3 : instrument not found message
part 7 : instrument not found message

If I don't get them in their respective slots - assigned to the right channel, the programs spins forever - and I have to end the task via Task Manager.

2015 installed over a working 2014.
I ran the Authorization Manager (which again required downloading an update saying it was "out of date") and enter the serial and code that I got originally in 2012...
This is going to "get old" very fast (like already!)

I tried the fix listed above - it did not change anything for me.

Thoughts?

150+ Songs produced using BIAB RealTracks
And 50 more...
and some more...