Hi,
having been successfully helped on importing a Trello board, I now want to use it !
I was able to add circle, groups and users, but then I seem to stay on an infinite loop:
Did I miss a good practice in including new users ?
I can’t help you directly, just in such cases it can be helpful to check logfiles:
https://docs.nextcloud.com/server/stable/admin_manual/issues/general_troubleshooting.html#troubleshooting-web-server-and-php-problems
They often reveal more details, with such errors, you can search for solutions and get more specific details about the problem. And then it is easier to help you or if the problem is unknown, create a bug report to get it fixed.