Hello,
In the last logs you just posted, we can see that Paperwork also used "brother4:bus3;dev2.
Anyway, if I’m not mistaken, “busX;devY” probably refers to your USB bus (you can probably confirm that with lsusb). And the last number may change if your device disconnect and reconnect, or if you reboot your computer. I assume this is what is happening here.
Unfortunately, Paperwork 1.3.1 isn’t really smart about that. If the exact device ID is not found, it doesn’t try to look for another device with a similar ID. So each time you open Paperwork, you have to open the settings first so it looks for scanners, and then you should be able to scan (… until the device ID changes again).
This issue has been fixed in what-will-be-Paperwork-2.0.
Best regards,