Got the same problem. I opened up a project that was previously created in Cubase 10 Pro. After the project has loaded, Cubase hangs with a spinning wheel and I need to force kill the application. This can be reproduced any time, no chance to do anything inside the project. I fixed that particular project by opening it in Cubase 10, just closing all of the plugin windows (but NOT unloading them!) and saving the project again. After that I was able to open it up in Cubase 10.5 with no crash.
But starting on a completely new project in Cubase 10.5, it crashes again after loading a third instance of the same plugin. It worked well on two tracks and after opening it in a third one, the same issue re-occured again. It was a completely different plugin, that was not loaded at all in the other project mentioned above, so I doubt that it has anything to do with a buggy plugin.
At this point I cannot use Cubase 10.5, as it is crashing all the time I didn’t have a similar issue with the previous versions since C6Pro.