AUSampler prevents me from opening GarageBand projects

Hi there, I'm pretty new to GarageBand so bear with me :)

I've been playing with soundfonts in GarageBand, using AUSampler. I can import soundfont instruments and play them just fine. Saving my project works fine, too. However, if I try to open my project after I closed it, I get the following error:

An Audio Unit plug-in reported a problem which might cause the system to become unstable. Please quit and restart GarageBand.

GarageBand then closes, preventing me from accessing my project. This seems to happen with any project that has more than one track that uses AUSampler. I also have Enable Audio Units checked in the preferences of GarageBand, so that isn't the issue. Can someone help me fix this?