A Debian user reported the following bug (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=357844). I don't know if it is feasible to fix it, but perhaps you can look into it.
If cookies are not enabled (either from the domain of the server running phppgadmin, or by default) then even if everything is configured correctly, a succesful login ends by taking you back out to the login screen.
phppgadmin should really check to make sure the client has cookies enabled and provide a warning message on the login screen if they are disabled.
This bug should probably be forwarded upstream. (I checked the latest upstream tarball, version 4.0.1, but it did not have a changelog and the SourceForge bug tracker had no open or closed bugs relating to cookies).