On the server who are you, and who does the webserver run as?

The fact that you set the permissions to 755 means that anyone can read or execute in that directory, but only if they are running as you can they write. I suspect that the webserver is not running as you.

Another basic sanity thing to check, put the name of the dbm file into a variable and include the contents of the variable in the debugging statement. I have spent too much time in the past trying to track down why X was not being created when in fact it was trying to create Y...