This seems to be a common problem with the Boxster. There is a guide bushing for the locking pin which pops out, you can get a replacement at
Pelican Parts.
With the bushing gone, there is a rubber loop for the pull-ring that will snake out of the hole and snag the guide pin. Mine was stuck in the open position so I just pushed it back in the hole and used a rubber grommet until I got my official guide bushing.
You might be able to use something thin like a feeler gauge through the fuel door to work the rubber loop back. Otherwise, you will need to start taking things apart.