However, sometimes cache plugin causes the issue. In this case I would suggest to clear the cache and deactivate cache plugin and check if the issue still persists,
If still the issue persists, you can check for other plugin conflicts. Just deactivate and other plugins except wpdm and premium package then activate one by one and check if the issue persists.
Also I found that you are using double download manager, just delete all and re install it from add plugin.