Hi, Andries.
You posted in our Pre-Sales forum from a user account that doesn’t have any purchases associated with it.
We don’t answer support questions for The Events Calendar core/free or Event Tickets core/free or their paid add-ons in the Pre-Sales forum.
However, I understand this is a bug report. In the future, please report such things via our free support forum for The Events Calendar.
For this one time, however, please do specify which URL(s) this error is happening on and we’ll see if we can replicate the issue.
Thank you.