Error accessing PostgreSQL

When I am trying to connect to PostgreSQL e get the following error:

ERROR:  pg_attribute_attrelid_index: cannot extend.  Check free disk space.

I should have about 98 MB of free disk space in PostgreSQL database file
system.  Is there any
way to check if there is a minimum amount of free space that must be
available ?

Could it be other problem than disk space?

Ricardo Maia