The error is caused by missing priviledges of the Flash file installed with Liferea. Beginning with Flash 9.0 there seems to be a stricter checking for Flash applets.
The problem will be solved in the upcoming releases 1.2.14 and 1.3.4. If you want to fix the problem yourself you can download the Flash Local Content Updater from Adobe and run the following command to add the missing priviledges:
$sudo ./LocalContentUpdater -a /usr/share/liferea/media/xspf_player_slim.swf
Please adapt the "/usr" path prefix to match your Liferea installation.
Much thanks go to tsukasa for pointing this out (German blog post).