Thanks @Timo. Are you able to create the template in that new project? If I fork, it works for me, although it is missing all the sub assets, which might be causing the issue.
I am getting the feeling it has to do with the fact that this entity was another template. I unlinked it in the hierarchy, modified it, renamed it and tried to create a new template from it. Somehow under the hood it seems to be connected to something.
There is a new child entity within this entity which is really big. I think Playcanvas rejects me from creating a template that’s this big. It worked for you @Mark_Lundin because the files where missing.