usually this error means the login session for your user has been timeout. But there are multiple reasons it can happen.
First thing is simple you have been too long on that page and session is timed out. In that case you can refresh that page in browser and try to upload.
Other things can include issue with your browser, or few extensions which are enabled which might be blocking the cookies. In this case try this operations in other browser and see if issue persists.
Did you check in browser settings in "disable cache" option is checked, that might also cause this type of issues.
Please check this posts out it might help you troubleshoot the issue