or-tools: Or-Tools running in IIS crashes

The C# program was running fine for quite a while; until re-installed on IIS … (as virtual-application, under Default Web Site).

Every time the app reaches the point:

model.AddDimension(manhattan_callback, big_number, big_number, true, "time");

it crashes with:

In VS getting: Exception thrown: ‘System.AccessViolationException’ in Google.OrTools.dll

We have seen it a few months back, and I recall the fix related to some 64 bit vs. 32 bit set-up, but cannot recall exactly what we did. There were also cases VS indicated “unable to start the web server”

Any tips?