I think I will try to glue it with super glue (until I got the replacement pot). I don't think it will hold permanently, but I'm curious to see what the glue can do.
I like to be critical of the service behaviour of audio gear companies, but you shouldn't just grumble, you should also mention positive examples.
I actually contacted Boss via Just music Berlin and asked for a suitable spare part and it actually worked. The part costs €10 and it took a while, but I'll be picking it up from the Berlin branch on Monday. Special thanks to the service staff at Just Music! It's absolutely worth mentioning these days when you still get service for a product that's over 10 years old. Bravo!