Chain tension and tensioning a chain

hippy It's often poor derailler adjustment or a bent derailler hanger combined with frame flex that drops the chain.
But, seeing as you were changing parts around it's also likely to be OLD + NEW = POO.
You should keep a chain and cassette together. It's not often that a chainring would do this unless the chain is well worn.
Do you know where did the chain slipped? Front or back? Did it drop the chain or just derail and 'autochange'?

No idea where is slipped. It's not 'autochange' either. It was slipping way more when I had the other chainset on it. Putting the original chainset back on seemed to solve all the problems, until now. BTW it's all old stuff not old and new.

Chain seems ok condition. It's probably the rear dr. I'll check it tonight.