Apart from the information request that HCP has mentioned, you could also hook your Android device to your PC and try to fetch logs for the python events that happen right as soon as it attempts to sign the transaction. I'm not sure if this would help pinpoint the issue but maybe could be a good starting point:

1) Turn on USB debugging mode from developer options.
2) Connect your device via USB and on your windows / linux machine open up a terminal / command line
3) Type in the following to check if your device has connected
4) Try to perform a logcat via adb, open up electrum on your phone and then attempt to sign the transaction once again. If you see any error post that in here. Do note while doing this if you see any sensitive information, do not post it. [Just incase]


Once you do this you'll notice a lot of python events being logged in your terminal / command line on your PC. Keep an eye on it when you attempt to sign the transaction.