Is the Meeting server federated with LDAP ie is it part of the cell and not a stand alone conversion/recorder/renderer server?
Have you put trace on the Meeting and Connections apps servers? I'd capture that trace as that's going to be what gives you a hint as to the 500 exception.
Do you see any exceptions at the time for example unauthorised users trying to connect to the deployment manager over the SOAP port or SSL exceptions?
Are the security sections for the ISC and SSC configured the same for each cell?