I could reproduce this with the config files from the HMA VPN service.
The config files can be downloaded from this location:
http://hidemyass.com/vpn-config/vpn-config.zip
(you have to delete the second line with 'ns-cert-type server' or else it won't work)
The config files have an inline <ca>, <cert> and <key> section.
The OpenVPN app for iOS can connect to the server.
Standard profile
If you delete the <cert> and <key> just like in the ovpn file downloaded from the Syno, you have the same issues:
External certificate profile
Two options:
Fix the OpenVPN app for iOS. 😃
-or-
Generate the client.crt and client.key on the Synology and use these in the config.
(and delete the 'client-cert-not-required' from the Syno server config)