I tried compressed air first, but that didn’t help.
Then I disassembled the module until I had good access to the sensor. The connections of the sensor the the servo PCB were fine, so I just gave it a good clean with isopropyl alcohol and a Q-tip.
There’s also quite a bit of play on the way the sensor PCB is mounted in relation to the moving part. I’ve tried mounting it as similar to the working side as possbile, close to the center. (It’s visible in the attached picture, the position of the sensor depends on where you put the screws in.)

And the Virtual Boy is working perfect again. Really glad is was an easy fix after all. Thanks RunnerPack!

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Yes, that’s 100% the problem I’m having! I spent a good while searching for similar issues but I didn’t come across that thread. It’s hard to find any troubleshooting information for the Virtual Boy besides the flex ribbon glue issue.

The right mirror oscillates at a fixed position, while the left one slowly moves in a circular motion as well.

Thanks for pointing me in the right direction, I will report back how it goes.