How to dismiss "This plug-in may require an update" warning

Like in this thread, if a Audio unit plug-in crashes during launch, GarageBand will block that plugin, presumably until a later version is installed.

An error has occured with an installed Audio Unit plug-in.

GarageBand unexpectedly quit while trying to use the Audio Unit “Magic keytar” by “Jan Hammer software LLC”.

This plug-in may require an update.

You may still desire to try to run the same plugin again, but the message (and the blocking) remains on each launch. Even when removing that plugin. Bad mojo.

The solution turns out to be simple, the plugin "blacklist" is stored in

~/Library/Caches/com.apple.audiounits.cache

that is, within the user's library folder. Simply delete this file (it's a cache, and will be regenerated) and the plugin can be run again.