Are you sure Wieliczka Salt Mine works as intended? Because over many campaigns I played, I had multiple cities which had been founded on hills and had mines (sometimes more than two), yet this wonder couldn't be constructed. I really can't figure out why, because sometimes it can be built and sometimes not. I'd say game has issues with recognising "places for salt". Also even after playing to late ages I can see some tiles were perfectly fine for spawning salt (no resources whatsoever), yet game didn't recognise it. How is it calculated exactly?
I have had something of the opposite issue with that wonder. I am able to build it in cities where there aren't places for salt, and no salt is added to the map when I build it.
Ok, folks, can you check if changing line 642 in UniqueWorldWondersRequirement.lua file fromSorta late, but I have experienced this with Ruhr Valley several times. (dedicated a couple-games to getting it actually), only for it to not be buildable despite having met all the conditions, and later discovering that there were valid tiles that did not have any resources on them. There were two instances in which it wasn't buildable in these very specific circumstances and both them involved several empty forested tiles. (they didn't have any Uranium or Aluminium on them either).
Code:
if pSpecificPlot:CanHaveResource(eCheckedResource, false) and pSpecificPlot:GetNumResource() == 0 then
Code:
if pSpecificPlot:CanHaveResource(eCheckedResource, true) and pSpecificPlot:GetNumResource() == 0 then

So it's really not so bad. Although a museum is +4 for the same cost, it comes so much later.