It's hard to tell from a photo, but the chain's pitch seems to be too large. This is classic "chain stretch" but it's really early a 300 miles.
Confirm "chain stretc" measuring with a 12" ruler. The pins at both 0 and 12" should line up perfectly (24x1/2" pitch). If the 12" length of chain is more than 12", that's wear which allows the links to move apart at the pins.
Question--- were you care full to properly tension the chain?
If yes, that might be the problem. A tensioned chain is an over tensioned chain, and will lead to rapid wear. Properly "tensioning a chain is about leaving the proper amount of slack in the system. Since sprockets are often eccentric, you want to find the tightest place and leave vesitlgial slack there - about 5-8mm FREE up and down play at the center.
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Chain-L site
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