Comment 2 for bug 918232

another possibility is that the there is an expired adb connection that remained by last job or last action,
and this cause the creation of new adb connection failed.

so it's better to clear the expired adb connection before create a new one.
this has been added in branch lp:~liuyq0307/lava-dispatcher/mount-data-partition.
and we can see after this modification whether this problem still occurs.