The runtime is reading the model coming from the Modeler (.mdp file) and encounters a string that is 43MB large! The maximum of the BSON library that we are using is 32MB.
It seems unlikely that you entered millions of characters yourself. Did you recently change something in the model that could cause this?
Probably best to handle this by submitting a ticket in the Mendix support portal. This is not caused by something you did wrong, but likely a Mendix bug.
So, reversing my adjustment did the trick and I can now run the model again. It seems that setting open a too complex part of our model created this error. I will chop up the operation in smaller bits, hopefully the operation then works.