@Jaap-Jan_Swijnenburg I’m sorry you’ve experienced an issue.
I’ll respectfully disagree with you there 
The old QnA plugin used standard Discourse custom field type casting, which is meant to handle casting that field to JSON. But that just doesn’t work in some cases (like this one), which is why you (ideally) need to build in format checks when you’re working with Discourse custom fields (particularly if you’re working with quite old code and data like this). I would suggest that that is what the Upvotes Plugin needs to do here, which indeed is where the immediate error is coming from.
You can use the data explorer plugin to check what you’ve got there.