I got a new Mac Pro and installed everything from scratch. FCS and all.
Now when I start FCP I get:

License Error
This installation is not serialized. Please re-install the plug-in with the original installer.

How can I find out which one it is? I used FCS Maintenance Pack to find it but all it tells me that all the plugins are "enabled"

FCP seems to work ok but I leaves me feeling a bit uneasy...

Any ideas? Anyone?

and yes - I went through every single effect plugin to see if the registration was correct - I go no errors...

Some plugins have their own licensing systems (e.g. FxFactory and Red Giant plugins) and those plugins must be installed by their respective installers in order to work. I would recommend removing all plugins by major manufacturers and reinstalling these from scratch, as these are the most likely ones to have a proprietary licensing system.

Remember that you can disable plugins in Plugin Manager to help you narrow down the offending one.

The plugin manager tells me that all plugins are enabled. I installed each one with the original installer and entered the licenses.

Just had another very strange issue. I tried to import a ProRes file to a new project: - unknown file... the file was made with compressor.

actually I take that back, the file was encoded with QT 7.
Compressor didn't work, the "submit" button was grayed out. I uninstalled Compressor and reinstalled it- now it works.

Still have the license error though.