CalDAV verification fails on iOS only

Hi all Hoping you can help solve this riddle. I can get CalDAV to work fine on two iMacs, but using the same server details and following cpanel's separate iOS instructions carefully, I can't get it to verify successfully on iOS for the same account. I've tried delete and repeat, restarted the device, checked passwords, urls, port 2080, but nothing seems to work. Any tips?