I have an ongoing project in Cubase 12 and today I discovered that I’d managed to delete the bass midi for the track. I could of recovered it from a backup but as I was wasn’t that happy with it I decided to rewrite it. Anyway, in creating the midi I decided to bypass the amp output of Modo Bass and use a plugin amp sim. I loaded Guitar Rig 5 went to browse a patch and Cubase locked and then shut down. Great I thought - it doesn’t like GR5. So I loaded the project from the latest backup that had the new midi, loaded AmpliTube 5, went to browse a bass patch and it locked up and Cubase shut down.
I’m a bit freaked out now as AmpliTube and Guitar Rig are not related and yet their inclusion is a no-no to Cubase at this time. I have another Amplitube already running for my actual guitar part.
I should point out that on closing Cubase when running this project causes a background crash (it has for a long time now - Cubase doesn’t like something I have loaded) so opening Cubase gives me a grey screen each time but I have learned to live with this as many previous projects have done the same and I’m used to it now. I don’t like not being able to open an amp sim though.
Several issues at the same time… 
First thing I would do is to check if, for a reason, either Amplitude or Guitar Rig 5 isn’t listed in the Studio > VST Plug-ins Manager > Blocklist panel, but I doubt it : they are both widely used and if there was a true incompatibility, it would be well known, already. If it isn’t the case, something went wrong, either with Cubase preferences, with your project file or even both…
- In the first case, I would try to relaunch Cubase in safe mode, using the Disable program preferences option, and either ticking or not the Deactivate all the third-party plug-ins one. At least, you’ll know if something is wrong with Cubase, at this point, with or without any amp sim.
- In the second one, I would try to start a new project and import tracks from the old one with the File > Import > Tracks from Project command.
Keep us informed… 
I guess that you have already checked if you have the latest version installed? For GR5, there are probably no more updates, but AT5 does get regular updates, and it is usually advisable to install them, as IK products in my experience tend to crash once in a while, but they do fix the bugs and later versions are mostly stable.
Else, upload a crash dump from “Documents\Steinberg\Crash Dumps” (if you’re on windows, if on Mac, I am out…), so we can analyze it.
@Grumbleweed, check over at IK, maybe give support a shout. From what I read, it’s a bit hit or miss. In the linked thread, a user temporarily moved their presets from their location, and that worked for them.
https://cgi.ikmultimedia.com/ikforum/viewtopic.php?f=5&t=29699
All my music programs are kept up to date (well the best I can) but I think the probem might be Modo Bass rather than the amp sim plugins as running Cubase without the amp sims and editing MB caused another crash.
However, everything did settle down again and I finished what I was doing so that’s good (but no amp sims used).
As a side note I also removed a Waves headphone studio simulator and I think the background crash on exit might have gone away.
Thanks for giving some thought to this and I’ll see if the headphone plugin is the source of the exit crash I always had and if proven, avoid it. (I used to think it was the Reason Rack plugin causing the crash but it’s not being used in this project).
If only life was simple 