I seem to have found a possible culprit that caused this same problem with my Scannable app (iPhone 6S on iOS 11.2):
I had scanned a load of travel expense receipts but waited to send them until I got home. That's when the above described problem started to happen. What I noticed was that I had also scanned my passport photo page. That had originally tried to register as a business card scan, but I converted it back to a normal scan. When I deleted that one scan, the rest would easily send by e-mail. Somehow the scan (size? format?) caused the tray to trigger the app to crash.
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