Scannable is really a dead app. Yes, still available, but undeveloped. I'm surprised that it is still available but see no evidence that it has been changed/developed for several years or that it has any support available.
At a guess, I'd say that something changed in the OS at some point which has caused the issue. But that's a guess. If it were me, and suffering the issues described, I'd find an alternative application and try something that is current.
My scanner has its own software which integrates well enought with Evernote.