When I close a project and load and other project X3b crashes.
I have to close X3b, restart X3b, and load the new Project. Then everything works.
I never had this Problem in the last 10 years.
I guess this is due to plugins from Waves and Izotope.
Has anyone else this Problem?

Oh boy... I had this problem with X2 and X2a. I simply could not reliably open a second project. And sometimes, the attempting to open the first project would crash.
 
The thing is, the problem seems to have completely gone away with X3a (and b). One of the reasons I'm so happy with X3 is that this behavior has gone.
 
So, I empathize, man, but I have no solution for you.

Can you shift open the second project to see if it's a certain plug that crashes?

If you send me the crash dump or CWBRN number I will be able to tell you why its crashing. 
My best guess is a VST3/VST2 mismatch. Do you have the option to migrate VST2 plugins to VST3 enabled as well as hide related VST2 plugins? If not you could be mixing and matching Waves plugins which they do not support.
 
Please see this thread

I have send in the crash-dump. ID 20316 and ID 20315
"R6025 - pure virtual function call"
It happens to Project saved in X3b and X3 and X2a.
 
It happens also by reopening THE SAME project after saving and closing.