For those interested, Fabio gave some more info on the issue here:
Fabio also answered some questions you may find useful.
Main points:
- It’s all about the number of unique plugins, a workaround is to use the same plugins many times if you can. Multiple instances of the same plugin do not count since the dll is only loaded once
- Some plugins use an abnormal amount of available slots (still waiting for someone to identify those problematic plugins)
- Some VSTs do not release the slots when they are removed from the project - some don’t even after closing the project.
This last one is new to us and it’s very serious imo. So when you experience something like that DO NOT open another project. Quit Cubase first
Hoping for a quick resolution and in the meantime some more info from Steinberg