Error updating event participation by invited attendees in calendar

Hello,
when people I invite to a meeting click on the lin to accept or decline the invitation, they land to a page that says
“An error occurred while updating the status of your participation.
Please contact the administrator directly.” (translation from italian…)

any hints on how to solve this issue ?
thanks!

I think it would be a good starter to tell us which OS, software versions etc., etc. you are using. It would also worse to check if the Nextcloud log file shows any information related to this action or what exact URL is being called.

Thanks
I am running Ubuntu 18.04.2 LTS, mySQL 5.7.26, PHP 7.2.17, Apache/2.4.29, Nextcloud 15.0.5

URL is https://[myserver]/index.php/apps/dav/invitation/accept/ohp3Doc6ce9zNkPHxxqdWVuFZcW82stXz7s5AQG5KkbvQuGF6EwfSWkGDiyb

A similar problem has been solved by making sure that each user uses his individual email address, as described here:

Nevertheless an open issue still exists which is related to shared calendar invitations:

unfortunately this is not the case on my side.
each of my users has a specific, different email address.

I have the same problem with one user, other users are working, and yes the Email is unique and o, the affected calendar is not shared …

Nextcloud 15.0.8

Thanks for help

Wolfgang