Sanyo VCR4400 Betamax doesn't play tapes

[ESigma25]

I don't like to necro my own threads, but tonight (after waiting weeks for my belt kit to arrive after ordering it) I replaced the two belts in my Sanyo VCR4400 and now instead of just doing nothing when you try to play a tape, it seems to be threading my tape improperly and not retracting it fully into the tape when I hit Stop. I notice that I have an idler tire that's very cracked, could that be the cause?

Weirdly, fast forward and rewind work absolutely fine, it's playing and retracting the tape into the cassette that cause problems. Attempting to play actually causes a loud high-pitched squealing sound so something is slipping and slipping badly.