Can you please try to delete the browser's cookies and let me know about the result. You can also try to restart the application pool and browse the project again.
If the issue persists, can you please let me know some details about when the issue started appearing and what are the exact steps you take to reproduce the issue.
In addition, can you please delete the error logs under the ~/App_Data/Sitefinity/Logs
folder and then please try to reproduce the issue again and if possible send the the files generated under the above mentioned folder. This might give us additional information on what causes the issue in your case.
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and vote to affect the priority of the items