Ah Pictures - looks like its been left outside a bit - with some parts a bit rusty - but thats easy to fix - wax / oil or replace - screws are low cost, even high quality ones from a specialist engineerimg fasteners supplier.
To improve the belt slip I’d check:
- Front bearing adjustment (like a BMX bike) … if these are loose the whole crank assembly is floppy loosing energy and belt tension.
- belt tension - which needs to be ‘just right’ … once all bearings are correctly adjusted, the whole plastic BB mounting can be moved up and down the tube by loosening the mounting screws, plus using the small M4 nut underneath to add tension (and , with this nut unscrewed, a soft mallet to reduce tension). I ended up experimenting and following explanations on this forum and in the manual. If the belt is too tight- it feels like its dragging and probably wearing everything out - if too loose and it will jump more. In wet and high powered starts a strong rider will always be able to cause tooth jump - even on the latest expensive carbon drive systems - if you read the reviews. So best I find is to push off and dont max it out in the wet and when setting off. That was why they introduced the snubber - which for heavier riders (like me) really works.
- Maybe the rear pulley teeth are worn down - check these as if really worn the rear wheel will need replacing - from Strida.eu
I hope this helps - have you got used to answering q’s about the bike when you ride it ? 