Check here! in details how to use the app.

This happens when the popup permission is blocked or u had interrupted the process that closes the permission to write the file on your disk ine online-mode.
To Solve this try below methods
1.Try enabling popups or remove popup blocker from your browser, you have to give permission for popups and redirects from the browser's site setting.
Check Here!
2.Use offline-mode to receive files

Make sure that both the sender and receiver is on same network.
Check here how to receive the files correctly.

This can happen if the files are being received in online-mode, as the browser is trying to ping service worker with each file request.
The pinging the service worker happens using iframe hence the popups.
You can try using offline-mode

This can happen if the receiving is interrupted somehow, try
1.Asking the sender to send file again.
2.If above did't work then the file might not be supported by your phone!

This can happen if the app is killed in the background or the app has crashed, to solve this
Open the app through the notification and the close it again, this will close the app properly.