Error message , error saving to custom config: open /etc/gitea/app.ini: permission denied

Hi there,
have a little strange behavior.
If the symlink to app.ini is missing in /etc/gitea, I get this error message when i push.

setting/setting.go:1168:CreateOrAppendToCustomConf() [F] error saving to custom config: open /etc/gitea/app.ini: permission denied

Gitea is started via systemd with these parameters:

[Unit]
Description=Gitea (Git with a cup of tea)
After=syslog.target
After=network.target
Requires=postgresql.service
[Service]
Type=simple
User=gitea
Group=gitea
WorkingDirectory=/var/lib/gitea/
RuntimeDirectory=gitea
ExecStart=/usr/local/bin/gitea web -c /etc/gitea/conf/app.ini
Restart=always
Environment=USER=gitea HOME=/home/gitea GITEA_WORK_DIR=/var/lib/gitea GITEA_CUSTOM=/etc/gitea
[Install]
WantedBy=multi-user.target

Why does this symlink have to be available so that you can push?
thanks for any help here.
Stefan