Thrown out of admin

We are currently running Zencart 1.5.5.a.

We are running on Centos 6.8, MySQL Version 5.5.57 and PHP 7.0.23.

We can login into Admin OK and can make changes however once we hit the "update" button we get thrown out of Admin and end up back at the login page.

On checking the database has updated and all aspects of the edits, deletes etc. have been actioned however we kept ending up back at the login page.

We have checked all recommended settings and using the settings recommended by Zencart.

We have troubled shooted the site and checked sessions etc. All appear to be working 100%

Any recommendations or assistance would be appreciated.

Thank you.

There are four sub-versions issued after what you have installed. While I don't see the above listed as a specific issue in the known ZC1.5.5 problems, I also didn't look at the full documentation of fixes for each sub-version to see if/which version may have corrected such an issue with sessions.

Would suggest identifying what was done to "check" the settings? What is being used as the expected value(s) and what are the current settings?

You didn't explain which "update" button/s is specifically triggering the problem. If it's from a plugin, it could be that plugin is written for a very old version of Zen Cart and doesn't have proper security protocols in place.
There could be a dozen other factors, mostly outside Zen Cart.

When did this problem suddenly start? How long has the site been "running fine without this problem"?
What's changed in the meantime, particularly with server-side software?

What's the URL? Usually if there are the kind of problems you report, then people also can't add things to the shopping-basket on the front-end of the site either. Let alone login.

Thank you for your response.

Firstly all aspects of the website are running correctly on the catalogue - front end side.

Clients can login and make purchases and check out without issue.

"What's the URL? Usually if there are the kind of problems you report, then people also can't add things to the shopping-basket on the front-end of the site either. Let alone login.".

There is no issue on the front end. Every aspect of the front end works correctly.

For clarification let me describe the "You didn't explain which "update" button/s is specifically triggering the problem".

Lets say we are logged into Admin. If for example we want to change say "Configuration" My Store "Store Name".

We hit the "edit" button. This works fine. Change the Name of the Store and then hit "update" we get thrown back to the Admin login page.

We log straight back in and then check if the "Store Name" has been updated. It has been updated.

Same for any "updates" in Admin section.

In terms of overall settings we have checked each recommended settings as per the Zencart install instructions and ensure these all comply.

The website was updated and ran once again perfectly for approx. 1 month.

The only change we can identify was an update during that time from PHP 7.0.18 to 7.0.23.

I hope this assists.

Thank you for your input.

Sounds like your admin session cookies aren't persisting properly. Test by deleting your browser's cookies and restarting the browser.
I wonder if your admin configure.php is from an older ZC version.