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There might be some issues with the app especially if it recently updated itself. Try going into your app manager and clearing the cache on Google Photos. if that doesn't work:
- tap on "disable" (I don't think you can uninstall it).
- It will uninstall all the updates (you won't lose any photos).
- Once that's done, tap "enable"
- Go to the Google Play store and check for updates
If you have ANY problems with these steps, come on back to this thread and let us know what you tried, and if there are any other errors. We'll do our best to guide you.