Plugin does not calculate any more

I’m building a website and I used this plugin to do some calculations for a product and it all worked like a charm.
We’re still working on the website but this morning we did some last tests and it looked like everything was ok.
But then all of a sudden we noticed that the calculations stopped working … I tried everything I could to find the bug, but I can’t find it.

I disabled all plugins
Reinstalled the plugin
Tried to create a new product
…

Nothing helps.

What can be the problem? And how can I solve this?

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