In a newly project (created with a kanban template) I cannot seem to add a child item (task, story or bug) to an epic. I am able to create tasks in the project, just not as a child item to an epic. I cannot drag them to be a child to an epic either.
When I try to add a child item to the epic, I receive the following pop-up:

I am a space admin, and even an organization admin.
I do not understand why I'm not able to do this, as I have a similar project where I am able to add child items to an epic.
This project uses the default software permission 'Default software scheme', the other project, where it's no problem uses another permission scheme, but I have checked and both of them have the same configuration.
The epic has a status 'in progress', the task (if I'm trying to move it under an epic) has the same status.
I did find this in the community; https://community.atlassian.com/forums/Jira-questions/Can-no-longer-create-assign-Child-tasks-to-Epics-August-6th-2025/qaq-p/3084973 but this doesn't seem to be the problem.
The default assignee of the project is 'unassiged'.