I made wrong conclusions when I first asked the question. It's not related to Console Shaper, it's the song itself.
I now found that the song does open reliably when I first create a new empty song, then open the song file in question. Works 100%. I assume with that broken song there's a problem opening the audio device maybe. I use an RME 9632 PCI card on Windows 10.
Made a complex song file with various softsynths and plugins. It all worked fine until I started to use the Console Shaper Mix Plugin on 2 busses.
Next times I tried to open the song Studio one would freeze while loading the song - not crashing, no "this program doesn't respond" message from Windows 10, it's just frozen.
When I then kill S1 from Taskmanager and try again to open the file it works sometimes, usually after the fifth to tenth attempt. Once it's open, the CPU bar strangely flickers between 40% (normal load of that specific template) and 100%.
Saving to a new file alone didn't fix the issue.
After I removed the Console Shaper (just picked it randomly cause I added it last) and saved again, I now can open the project everytime without issue.