It shouldn't be an OS difference.... I mean I'm also running Windows Mobile 6.1 and the rom I'm using (latest official one from Samsung for portuguese Omnia devices) is from December.
A harder approach... Try the following:
1- Make sure all is ok with your .NET intallation (unninstall&reinstall if needed).
2- Same thing with WVD.
3- Unninstall any TCPMP and pluggin for it you have. Reboot if needed.
4- Check with the file explorer and delete any leftover stuff in the program folders that relates with TCPMP. Reboot if needed.
5- Check the registry and delete any TCPMP key. I think keys are in HKEY_LOCAL_MACHINE>Software (delete the TCPMP folder). I use FDC TaskManager
6- Soft-reset device.
7- Either try TCPMP GF5500 4.1
or retry what I wrote some posts ago (installing TCPMP 0.72RC1 recomp3
and the flv streaming pluggin
8- Try to open an flv1 file offline with TCPMP (download it from the web using your desktop or laptop and put it on your device)
9- If point 8 is successfull try using WVD
10- Even if you get a crash report when trying to play you sometimes are able to go to TCPMP and it'll play the flv file nevertheless (just go to the TCPMP task and ignore the error reports).
If this doesn't work you can always try to play flv1 files on your drive and watch videos from YouTube using youtubeplay
If all fails the problem may be elsewhere (maybe the rom your using if it was cooked with bugs... you can always try a hard reset... don't really know) or you're simply out of luck with your device...