Problem reading 'inbox' (launched-with) files on iOS 5.1?

[Ben Rubinstein]

I'm developing an iOS app that accepts files from other apps.  When it 
receives the 'urlWakeUp' message with a suitable URL, it reads in the file, 
checks it's the right kind of file, and then stores it in the user data folder 
for subsequent viewing.

This works fine on my iPhone 4s under iOS 8.4; and on an iPad 2 under iOS 
7.1.1; but on an iPad 1 under iOS 5.1.1, it fails.

Specifically, attempting to load the data using
       put URL tSrcURL into tData

yields empty and the result is "can't open file".

The app can list the 'Inbox' folder and see the details of the incoming files, 
complete with their size etc; but can't read them.

Was there a known issue in relation to iOS 5.1?  Is there a workaround?

Many thanks,

Ben