How to fix "Couldn't obtain plugin principle class"
RapidWeaver Classic is a Universal app and compatible with both Intel and Apple Silicon-based Macs.
To take advantage of RapidWeaver on Apple Silicon based Macs, you’ll want to make sure all your plugins are up-to-date. If some of your installed plugins are not Universal, they will not run natively in RapidWeaver on Apple Silicon.
If a plugin isn't compatible with Apple Silicon you will see the following error:
RapidWeaver Failed to load some plugins
PluginName: Couldn't obtain plugin principle class
To make older plugins (that are not Universal) run on an M1 Mac you'll need to install Rosetta for macOS. Apple has a guide on how to install Rosetta. Along with information on how to enable it on a per App basis.
However, by running RapidWeaver under Rosetta, you'll lose some of the huge speed improvements M1 chips can bring. Please check with the developer of any plugins you're using to see if they are available as Universal builds. If they are not available we'd suggest looking for alternative plugins and stacks that can cover the features you're missing.