Hi Dimitri,
we had the same issue and resolved by deleting a widget called PrintPDF.
In your case it might be some other widget, please check you last working version and see if what are the new widgets added from that version, then delete and try again.
Hope it resolves your issue,
Regards,
Ashok
Same problem here! Also only non-persistable entities – Mx Studio Pro 9.9.1 and Mx Studio Pro 9.10.1. We have fixed the error by removing the PrintScreen widget from our project.
Same Problem.