since there is no exchange left - can anyone tell where the weird price-peaks come from?
I had to do some research to understand your question and I am still not sure I got it.
Coinmarketcap doesn't have HeavyCoin.
WorldCoinIndex has it.
There are other coins nowadays with HVC ticker, I don't know if any of them made it to exchanges.
The only place I've seen a spike was on WorldCoinIndex on HeavyCoin vs USD. I guess it's a bug and it's more related to Bitcoin vs USD price than anything else. The HVC-USD pair for the same period has no spike. And there are no exchanges listed either.
If this didn't answer your question, please be more exact when asking.