Hi Walter Nordmann,
I am assuming, you're using the linux.
Please check the log files for the pgAdmin III (i.e. ~/pgadmin.log) for any clue for failure.
The last option is to rename the ~/.pgadmin3 (settings file for pgAdmin III) on any linux environment.
NOTE:
- This file contains the settings for pgAdmin3. You may loose existing server information.
- The latest database server may not work with older version of pgAdmin 3.
-- Ashesh Vashi