I had the same issue. This helped me: Markdown app doesn't work after update to NC17
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