Magento 1.x Backend Error: 'Class Magento\Backend\Model\Auth\Interceptor does not exist

[jcooperoff4749]

Hi all,

I'm currently encountering a recurring issue in Magento 1.x where I'm unable to access the backend login page. Instead, I receive the following error:

Fatal error: Class 'Magento\Backend\Model\Auth\Interceptor' does not exist in /path/to/magento/backend/...

I have tried multiple solutions like clearing the cache, reindexing, and even checking for corrupted files, but the issue persists sporadically. It seems to appear after a certain time without any noticeable trigger.

Some specifics:

Magento Version: Magento 1.9.4.3 (CE)
PHP Version: 7.x (I suspect compatibility might be an issue here, but not sure)
Custom Modules: Several custom extensions and a couple of outdated ones which might be interfering
Steps Taken: Cache flush, reindexing, checked the custom modules for possible conflicts, reinstalled the Magento core files.
Has anyone encountered this particular error with the Auth Interceptor class? Could this be a conflict with custom modules, or is there something wrong with my Magento configuration?

Any guidance would be appreciated!

Thanks in advance.

[michalskis8e53]

• This error usually appears when your Magento installation is referencing a Magento 2 class (Magento\Backend\Model\Auth\Interceptor), which does not exist in Magento 1.x.

• Magento 1.x and Magento 2.x have very different structures and namespaces; this error suggests code meant for Magento 2 is running in a Magento 1.x environment.

• Check if any extensions or modules recently installed were built for Magento 2 but mistakenly installed in a Magento 1.x store.

• Inspect your app/code/ and app/etc/modules/ directories to verify if any non-Magento 1 modules were added accidentally.

• Search the codebase for any reference to Magento\Backend\Model\Auth\Interceptor and remove or replace it with Magento 1 equivalent logic.

• If the error appeared after using a yes or no button installer or wizard, double-check what dependencies it added — it might have brought in incompatible Magento 2 code.

• Revert any recent changes or deployments that may have introduced the Magento 2 class into the Magento 1 project.

• If using Composer (rare in Magento 1), verify your composer.json doesn’t include Magento 2 packages.

• Clear Magento’s cache (var/cache/ and var/session/) to remove any stored references to invalid classes.

• Enable Magento's developer mode to get a more detailed stack trace and help identify the source of the error.

• If the issue persists, try disabling custom modules one by one to isolate the one causing the error.

• As a long-term fix, consider auditing the entire codebase and ensure no Magento 2 code or namespaces exist in your Magento 1.x environment.