I have made some tests and my findings are described below.
I cloned the qtsvg project from the gerrit server and copied it into my local gerrit site and could verify that the footer settings couldn't be modified.
I then noticed that when initializing a new project and copying it to my local gerrit site the same problem occurs. However, it does not occur if the project is initialized by gerrit by running ssh -p <port> <host> gerrit create-project --name <NAME>.
When comparing the project.config of qtsvg and the project created by the gerrit command, everything is identical except for the top 2 lines:
owner = group Administrators
This seems to be put there by gerrit when creating a new project.
When adding the Owner permission for the reference refs/* and group Administrators to the qtsvg on my local gerrit the problem was solved. The All-Project inheritance doesn't seem to work for this matter though.
Maybe the qtsvg wasn't properly created in gerrit? Does it sound like a proper solution to add the permission mentioned above to the projects with this problem?