My guess, judging by the crash logs, could be conflicting screen names (maybe some URM mod screen coincidentally uses the same screen name my game does somewhere). But I'm not sure. They're the ones who should investigate & fix it.
So, the game was always meant to be a Sandbox, but it also has a story to be told. It is much more efficient and easy to lay out a linear prologue with all the important story beats before letting the player run wild, or the sandbox world could feel shallow, confusing or restrictive depending on the approach. I assume other devs do it for the same reason. Even big games, like Kingdom Come Deliverance, are structured exactly like that.
I guess my prologue was big enough that people felt tricked into thinking it was a kinetic novel, but be assured, the full story's scope would be almost impossible to be told well linearly :P. That's why it's such a big prologue.