This was a known problem with versions older than 1.8, in which case deleting and re-saving the license will help.
In newer versions, this could occur due to problems with the encrypted storage of your license. Also in this case it should help to delete the license and save it again.
If this does not work, please activate the option „Advanced / Alternative Secret Key encryption“ and try it again. Important: After activating this option, you also have to save your Secret Access Key again on the Setup page.