I had - and unfortunately have - this problem often - at least if I understand your issue correctly.
It is (in my opinion) an issue with the VST Bitbridge.
When I open a project which was created in Nuendo 32bit in 64 bit and I have a lot of 32bit plugins in it it will hang while loading. In the same time it can happen that you “max out” the amount of 32 bit plugins during the mix and then you can not insert another one (of these).
2 years ago as I was working on a 400 track mix I went 64bit and had to spend some time fiddling around which plugins are fine with the bitbridge and which one do better when bridged via jBridge.
I ended up bridgning all UAD plugins and all those freeware oldscool VST2 plugins from “back those days” - while Drumagog for example was not running fine via jBridge, this plugin I use via Nuendo bitbrige.
Well, it is a PITA that there are still so many issues… Slateplugins confirmed that Trigger is still 32bit, though they have an 64bit installer… ?! Because I asked why I can not load that plugin in larger sessions (sessions using RAM more than 3GB or something)… It seems that the installer installs someting in the 64bit folder which is still 32bit and which is bridged by the host applications bitbridge automatically…
Today I tried to open a project in 64bit which was running fine yesterday - it hang up while loading. 32bit Nuendo opens whithout a problem, so does Nuendo 6. Nuendo 5.5.6 does not.
I removed ONE instance of drumagog - now there was just one single 32bit plugins left (a reverb) - I was able to open it. In the same time I am working on some Blind Guardian style mix, 400 tracks and zillions of plugins - for example 8 Drumagog and 3 of these 32bit Verbs… No problem!! I do not understand …
Try only to use true 64bit plugins. Does it now work? As I said - I have the feeling the Steinberg Bitbridge is only capable to run a certain amount of plugins… Roughly I would say 20 or 30 - but the issue today was differnt, just a few 32 bit plugins and I was not able to open anymore…
Brandy