It seems that some users intermittently get error 59920 "missing session string" reported when trying to do a proxy login.
When this occurs, it happens regardless of which account they try to proxy into.
I have not been able to narrow it down any as to what causes this.
I believe it is a defect of some kind in the POA.

These users can login over soap fine themselves, and can proxy to other users in the GroupWise Client.
If this happens to a user, I found the only thing which helps is to stop and start the POA.
If I replay the same soap messages which failed, after restarting the POA, they succeed.

I've encountered this on two production POAs which are both running 2014 (14.0.1) build 117687.
I've done a few days of automated testing with multiple builds and have not been able to get this error to occur in a test environment.
I've got the following versions in my test environment:
14.0.1 - 117687 (the highest build I have) & 117118
14.0.0 - 116215 & 116018
12.0.3 - 116810