Chain slipping on middle chainring
I expect that after 10,000 km your chainring also needs to be replaced (frankly, I find it astonishing that you made it that far without replacing anything else.  That's amazing.).  Am I correct in guessing that you spend most of your time in your middle chainring?  If so, it will be significantly more worn than your large or small rings, which would explain why you're only experiencing slippage in that ring.

It should be very easy to tell if it needs to be replaced--either take it to your shop and have them look at it, or look at it yourself.  Worn chainrings will show a characteristic "shark fin" on the teeth if they're worn too badly.