Okay, thanks for checking. Meanwhile I found out the root cause. The app is no longer available in the NC app store. Therefore I opened an issue in the app’s repository:
While there is no way (or at least I couldn’t find any) to install the app from the GUI, there is only a manual way to get the app working again.
You can perform the following steps. Please make sure to change directory into the correct folder. I just used a placeholder. Furthermore make sure to use the correct user (web server user like “www-data” or “http”) for the chown command.
Easiest way is to run these commands as user root. If you don’t want to or can’t use sudo before these commands (sudo cd ...).
cd /path/to/nextcloud/apps
wget https://github.com/nextcloud/files_texteditor/archive/v17.0.0.tar.gz
tar -xzf v17.0.0.tar.gz
mv files_texteditor-17.0.0 files_texteditor
find files_texteditor/ -type f -print0 | xargs -0 chmod 640
find files_texteditor/ -type d -print0 | xargs -0 chmod 750
chown -R <webserver user>:<webserver user> files_texteditor
rm v17.0.0.tar.gz