I had the same problem and used the same workaround - baanlogin instead of rexec. Portingset 9.1b.01.
What sucks is that I could not find out what was causing this error. Rexec is not prohibited, so why this message ?
Even more: I have this phenomena on two different machines and other servers with the same portingset dont have it.
Indianidol, if you found more insights about it I would be interested to know !