The issue mentioned in the GMC topic refers to an issue with for loops whose first statement isn't a variable assignment causing this error, I went through all for loops in the engine and none of them has it... only suspect thing is compatibility script __global_object_depths() having a "var i = 0" assignment, not sure if that would be a candidate with vars behaving different from ordinary variables? (The solution if this is the case would be moving the "var i" part outside the loop and then do a normal "i = 0" inside the for loop).
Geez, GMS2 still kinda feels like it's in beta sometimes :/