Error after upgrade to Joomla 6

After a Testupgrade to Joomla 6 Site crashed.
- Just Gridbox and RSJoomla Firewall installed.
- Backward Compatibility Plugins deactivated - after disabling site get an error Class "JPlugin" not found

Is this a general Joomla Problem or with 3rd Party extensions

A fresh Joomla 6 and Balbooa Products are no Problem. -> Great Job ^^

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