Usually when accessing pages in the backend, this is caused by a configuration issue in the environment or incorrect registration of the handler mappings in IIS.
The following KB article provides information and references on :
Accessing pages in the Sitefinity backend throws 404 error
Some additional checks you can perform:
- can you see the Staging and Syncing page under the Administration tab?
- or accessing it by manually typing it in the browser: Sitefinity/Administration/Synchronization?
- are the backend pages for the Site sync module present under Administration -> Backend pages
(Sitefinity > Administration > Tools > Staging & Syncing
If you see the backend pages there the issue must be related to the handler mappings, for which I refer to the forementioned KB article
I'll include also this forum thread that also handles about a 404 error when accessing backend pages;
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