The multiple Google accounts login issue
The upload error can be caused when you are logged in with two or multiple google accounts. This is a known Google accounts bug that they won’t fix.
When you are logged in with multiple Google accounts add-ons cannot work properly.
To workaround this issue/limitation, you can add different profiles on your browser and switch the profile to use add-ons with different Google accounts.
Please make sure you are logged in with only one Google account or switch the profile in your browser as above.
Login back only with the Google account where FileDrop is installed.