Ok, so I talked to Epic and tried a few things, and it seems the problem comes from one of the engine version upgrades in the past. Apparently due to these upgrades the BP function library in the Modular Menu System does not work in standalone mode any longer. And since no error is thrown, we were not aware of this issue. So unfortunately it seems now that we need to re-create the whole BP function library. That is quite a number of functions there, so I am afraid it will take a few days until we have re-created everything …
I am sorry, but we are working on it!