I don't know the cause, and am assuming -- for lack of information about your OS -- that the question deals with a Win version.

I have a similar issue (w/AS 5.012 and Win 7) if I invoke ppm without arguments at the command prompt: ppm4 goes to its GUI mode in that case, and fails with a 500 or a 'bad hostname' message.

Two workarounds: add bribes or other repositories to your repos OR use the command line version ( c:\<ppm help ) for starters

... OR do both.