Products get added with double quantity

Hi, I love this plugin and we use the Pro Version.
The PPOM enhanced products work fine.
But we have a problem with our cart on normal products without PPOM.
If I add a normal (non-PPOM) product from its product page, it gets added with double quantity. So if i add quantity 2, on cart page it shows “2x product xy got added”, but in the cart it has quantity 4.
The error stays, even with standard twenty-one theme.
If i deactivate “PPOM Pro” plugin, the error stays. Only if i deactivate normal “PPOM” plugin, the error goes away.
We are on WP 5.7.1 and i deactivated every other plugins.