The problem is, again, that when I only remove the wallet.dat file and I insert the old wallet.dat backed up file, it still states the password is wrong.
When I am completely certain the password is correct...
I can't work out what is wrong then. Try using btcrecover
https://github.com/gurnec/btcrecover If that doesn't work, your file has probably become corrupted and you will need an expert to have a look at it and see if it can be repaired.