Oh well, I spoke too soon
. While I tested it several times successfully, it now seems to still fail.
I run install.sh as well as remove.sh manually and wouldn't see any message. If I run remove.sh -upgrade before the installation through the web UI, I wouldn't get the error message. Thus it's pretty clearly an issue in the remove.sh script. I _think_ it's related to restarting the httpd daemon.
When I run the initial wizard to create the basic addon, I told it that no server restart would be required. But the remove.sh still ended up with that line. When would httpd need to reload its configuration? Our server doesn't use it - shall I just leave it alone? Or will frontview always require the server to restart, as the add-on is adding some new virtual folder for its simple settings page?