FIXED: I did several things without really testing if they work until the end, so just try them in this order. Test in a custom game to avoid afking.
1. Play without a browser (firefox, IE) open.
2. Install or repair Microsoft Framework 3.5
3. Log in using a different LOL account and load a game, the error may be caused by a profile glitch (had this crash issue on multiple computers) and loading a working profile appears to solve the issue.
You may then play using your original account.
Note that this only fixed the "crashing on field start", not crashing while actually loading. It may work to do that as well, but I can't test that.