Hi,
I have the same issue. I downloaded the Moodle Mobile app for iOS to test and I have the same error.
I did the test on our test server, it has Moodle 2.4.2 installed. I enabled the mobile services option in Settings > site administration > Plugins > Web services > Mobile. It didn't work so I also enabled the same option in Settings > Site administration > Plugins >Web services > External services. And I also checked that the XML-RPC protocol was activated.
But nothing, I still get the error "the site must use Moodle 2.4 or later", but I'm using Moodle 2.4.2 so it should work, or at least not telling me to use Moodle 2.4 if I'm actually using it. I tried with a user using LDAP authentification and with another one using Manual authentification.
So if someone has any ideas, it would be nice.
Thanks a lot.
GK.