Riiiight, cheers for the help guys. But apparently the solution was (ridiculously) easy.
I just went to that particular file (C:\Games\World of Warcraft\Data\enGB\backup-enGB.MPQ) and deleted it. The patch updater automatically made a new one and everything seems to be working fine now.
Imo a strange problem with an even more stupid solution but ah well. It works.
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