Reported on the 'Moodle for mobile' forum (https://moodle.org/mod/forum/discuss.php?d=397771#p1604873)

If a site has already enablemobilewebservice defined in config.php, the site admin can click on the checkbox of "enable web services for mobile devices"(Site administration >> Mobile app >> Mobile settings). (It doesn't save which is good). I don't think it should let the user click on it at all.