We have had that same problem while installing a beta version of 7.5. Do you happen to use a Domino server in an exisiting domain? If so, try the following:
- setup the Domino server as a first server, not an additional, using the id file created in the exisitng domain
- install Sametime 7.5
- replace names.nsf with a replica of names.nsf from the exisiting domain
Although not a nice solution, it worked for us.