An error occurred while sending the questionnaire

Just before we upgraded to NC28, we noticed that the Forms application wasn’t working as it used to. When I fill in the questionnaire and try to send it, I get an error that the questionnaire cannot be sent.
Could someone help me with a suggestion on where to start looking? We have an IT professional, but he’s not an expert on Nexcloud, so I have to dig for some information about it.

Upgrading to NC28 hasn’t solved the problem. And we have the latest version of Forms.