UPDATE:
Webkit playback will not work on Windows XP:
http://wiki.plexapp.com/index.php/Frequently_Asked_Questions#What_are_the_main_differences_between_the_Plex_Media_Server_platforms.3F
Unfortunately this means that Voddler will not work on your setup!
Ok, that's too bad. Thanks for the info!
UPDATE:
Hmm - my Plex Media Server.log on the other hand says
May 18, 2013 17:23:38:828 [1064] DEBUG - WebKit supports Flash content
May 18, 2013 17:23:38:828 [1064] DEBUG - WebKit supports Silverlight content
May 18, 2013 17:23:38:828 [1064] INFO - Successfully initialized WebKit
Then when trying to play
May 18, 2013 23:21:33:843 [1064] DEBUG - WebKit successfully loaded http://www.voddler.com/playapi/embedded/1?videoId=2721821284152501569&session=cee81422f883786cd511711ad0eae106&format=html&plex=1&wmode=opaquew&quality=autolow&width=1280&height=720
...
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: 7FF2AE10 Plugin loaded [player.voddler.com/VoddlerPlayer.swf] (8 8 1280 720 player)
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: Frame was loaded, processing event.
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: Executing Moving to state playing.
Here's my output from Plex Media Server log when trying to play - can you please compare with yours [member='meo']
May 18, 2013 23:21:33:687 [2044] DEBUG - [com.plexapp.plugins.voddler] HTTP reply status 302, with 0 bytes of content.
May 18, 2013 23:21:33:843 [4404] DEBUG - Request: GET /video/:/webkit?url=http%3A%2F%2Fwww.voddler.com%2Fplayapi%2Fembedded%2F1%3FvideoId%3D2721821284152501569%26session%3Dcee81422f883786cd511711ad0eae106%26format%3Dhtml%26plex%3D1%26wmode%3Dopaquew%26quality%3Dautolow%26width%3D1280%26height%3D720&prefix=/video/voddler [127.0.0.1:38308] (3 live)
May 18, 2013 23:21:33:843 [4404] DEBUG - * url => http://www.voddler.com/playapi/embedded/1?videoId=2721821284152501569&session=cee81422f883786cd511711ad0eae106&format=html&plex=1&wmode=opaquew&quality=autolow&width=1280&height=720
May 18, 2013 23:21:33:843 [4404] DEBUG - * prefix => /video/voddler
May 18, 2013 23:21:33:843 [4404] DEBUG - WebKit: [voddler.com/playapi/embedded/1?videoId=2721821284152501569&session=cee81422f883786cd511711ad0eae106&format=html&plex=1&wmode=opaquew&quality=autolow&width=1280&height=720]
May 18, 2013 23:21:33:843 [4404] DEBUG - Found corresponding plug-in: C:\Documents and Settings\Owner\Local Settings\Application Data\Plex Media Server\Plug-in Support\Data\com.plexapp.plugins.voddler
May 18, 2013 23:21:33:843 [4404] DEBUG - WebKit: Resetting to state fetch-player.
May 18, 2013 23:21:33:843 [4404] DEBUG - Using C:\Documents and Settings\Owner\Local Settings\Application Data\Plex Media Server\Plug-ins\voddler.bundle\Contents/Site Configurations\voddler.xml with score of 22.
May 18, 2013 23:21:33:843 [4404] DEBUG - WK_BrowseToURL http://www.voddler.com/playapi/embedded/1?videoId=2721821284152501569&session=cee81422f883786cd511711ad0eae106&format=html&plex=1&wmode=opaquew&quality=autolow&width=1280&height=720
May 18, 2013 23:21:33:843 [1064] DEBUG - WebKit successfully loaded http://www.voddler.com/playapi/embedded/1?videoId=2721821284152501569&session=cee81422f883786cd511711ad0eae106&format=html&plex=1&wmode=opaquew&quality=autolow&width=1280&height=720
May 18, 2013 23:21:33:843 [4404] DEBUG - Setting low latency option for two-way stream.
May 18, 2013 23:21:33:843 [4404] DEBUG - Beginning read from two-way stream.
May 18, 2013 23:21:33:843 [4880] DEBUG - WebKit: Sending line: HELLO
May 18, 2013 23:21:34:437 [4272] DEBUG - Request: GET / [127.0.0.1:38309] (4 live)
May 18, 2013 23:21:34:437 [4272] DEBUG - It took 0.000000 sec to serialize a list with 8 elements.
May 18, 2013 23:21:35:406 [1064] DEBUG - Plugin loaded (application/x-shockwave-flash, http://player.voddler.com/VoddlerPlayer.swf, 7FF2AE10, 8, 8, 1280, 720, player)
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: 7FF2AE10 Plugin loaded [player.voddler.com/VoddlerPlayer.swf] (8 8 1280 720 player)
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: Frame was loaded, processing event.
May 18, 2013 23:21:35:406 [1064] DEBUG - WebKit: Executing Moving to state playing.
May 18, 2013 23:21:36:500 [1960] DEBUG - [com.plexapp.plugins.voddler] HTTP reply status 200, with 0 bytes of content.