I had an issue with a number of software instruments - especially synth pads - showing the following error message when selected:
“Plugin Hybrid Mor cannot be found”
This seemed to run through a number of instruments and certainly wasn’t welcome, so I contacted Apple Support and ran through a series of checks to correct this.
Eventually, the solution was fairly simple:
1. Reinstall Garageband
2. Then run a software update for Garageband.
The issue comes up when files under the Hybrid Morph folder disappear.
Why this happens isn’t easy to fathom, but I’ve had applications not fully install with the OS X boot disc so I guess it’s just one of those gremlins that can come up.
You may be able to install just Garageband from the boot disc, but if you don’t have the relevant permissions, just re-install all applications and you should be fine from that.
Worked for me, so hopefully this works for you. 