A logout loop = logged out again immediately after logging in. Common causes:
1. Strict cookie blocker — explicitly allow cookies for `senly.io` and your portal domain
2. Privacy extension (Privacy Badger, uBlock, Brave Shields) — add Senly to whitelist
3. Wrong device time — sync system time; cookies have time-sensitive validation
4. Cookie conflict between domains — clear all cookies for `senly.io` + your custom domain
Does it work in incognito? Then the problem is in an extension or cookie setting of your normal profile.