I know you said it’s not your plug-ins but it probably is your plug-ins.
Can you create an empty Project? If you can, go to your Project plug-ins folder and see what is blocked or black listed. Find out where these plugins are located, go to that folder and then delete them. These would be VST2 plugs or 32bit plugs. They can’t be used by the current Cubase.
If you are using some type of adaptor like j-bridge (that tries to alter the 32bit plugin to work in a 64bit environment) then this is probably your problem.
If you are saying ALL your plugins are 64bit versions, then again, I would say that something is wrong with a particular plugin. When Cubase doesn’t.open it’s usually caused by a bad or mismatched plugin, this has been my experience.