Hello,
Home → Admin Zone → Setup → Configuration → Security options screen, do you have Enforce IP addresses for sessions ticked? This options, I think is set to 'TRUE' by default. I had the same issues sometime ago, and it happens if your ISP changes your IP address between connections.

Here's the text from the help tooltip:


Whether sessions should be tied to IP addresses. Enable to increase traceability and security. Users with erratic connections (e.g. frequent changes of wifi zones), and users sharing accounts, will keep losing their secure logins if this option is enabled. We recommend that you keep this option enabled on production sites.
Default: Yes

Hummmmm I will check that just in case but I seem to remember this option and I should have it set to false (off) but hey anything is possible with change.

Assuming it isn't the IP address.

It sounds like the session cookie isn't working properly.

Try using an incognito window. If everything works fine in there then that suggests the problem is a duplicate cookie under a different path/domain combination. Modern web browsers allow you to inspect and erase individual cookies.

If this doesn't help, check the cookies really are saving by using the tools in your browser.

Check the configured cookie domain and path are valid (they're in _config.php).