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Looping logins are normally always related to Firewalls, if you have a firewall, your XOOPS site needs to be added to the trusted sites list.
Other poeple though have found other reasons, some related to sessions, cookies, and phpini configs
https://xoops.org/modules/newbb/viewtopic.php?topic_id=17690&forum=4&viewmode=flat&order=ASC&start=20
Some servers need 777 permissions rather than 666 on the appropriate files/folders like "cache"