Sorry, I found the solution in the FAQ: this solves the issue:
I am unable to launch the application. I am getting a 404 error in the browser. What should I do?
Log in to the Synology machine via SSH as admin.
Run the ‘sudo su’ command and enter the admin password.
Run the find command 'find /usr/local/ -name IDrive.conf'
Find command will display the results like the following example:
/usr/local/etc/httpd/sites-enabled-user/IDrive.conf
/usr/local/etc/php/fpm.d/IDrive.conf
Remove the listed items by running the 'remove' command (rm) as below:
rm /usr/local/etc/httpd/sites-enabled-user/IDrive.conf
rm /usr/local/etc/php/fpm.d/IDrive.conf
Make sure that you have removed all the IDrive.conf files.
Restart the 'Web Station' package