Maybe a solution, maybe not:
We had two problems on a Windows Server 2003. One is the discused Problem with Zend Enabler the second is a connection problem to an oracle database that occurse sometimes. We worked on both problems seperatly. For the Zend Problem we testet many solutions, incl. increasing the Threads. Nothing worked. On the other side we changed some registry values to fix the oracle problem. Since then the zend enabler problem didn't show up again. Maybe Zend had have the same problem as the oracle instant client.
The problem for the instant client was, that we had to many processes for a to small desktop-heap. We just increased the desktop heap for not interactive clients:
Regkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\SubSystems