Thanks Eirik,
Maybe the logs on the devices will provide some info. Can you install the following app on one of the device:
https://play.google.com/store/apps/details?id=com.xtralogic.android.logcollector
Reproduce the issue and then run the app. It will collect logfiles and let you email or save them them. Once you have collected them can you attach them?
Richard Laos