The error messages suggest that there may be a problem with ToElementMesh in one of the following:
Needs["NDSolve`FEM`"];
ToElementMesh[\[CapitalOmega]]
ToElementMesh[NumericalRegion[\[CapitalOmega]]]
I can't check V11.3.0.0, but you might see if either produces an element mesh for you. If neither does, then that may indicate where the problem is.
Alternatively, you could try coding the region Ω as a Rectangle[] or a Polygon[] instead of as an ImplicitRegion[].
That it is working in later versions suggests it might be that there was a bug which has been fixed.
