Easy to “fix.”
Locate your various plugin folders, work on the largest one first.
Just temporarily move half of them out. Check if the project loads. If so, the problem is with a plugin in the half you moved out.
If it doesn’t load, move half of the remaining out.
So on and so forth until you find the one plugin that’s not playing nice with Cubase.
Then, make sure you have the latest version of that plugin and/or reinstall it. Try again.
If it still won’t load with that plugin, move the plugin (again), load the project, replace all instances of it (shows as “!!!name of plugin!!!” in Cubase) with some alternative, or nothing. Save project, reboot and try opening it again. Should be “fixed” so to speak.
At any given point in time, there will be some plugins in the world that won’t work with a given version of Cubase.
Lol, about 6 months ago there was one plugin developer (who I won’t name) that literally did not ever test his plugin in Cubase. It wouldn’t remember its settings in any version of Cubase. Hahaha.
I called him on it, in a forum, and he acknowledged it and fixed it about 2 months later (which is pretty awesome).