I have the same problem and its bugging me like crazy ... I recently added some flv files to check if i could play them . I tried kmplayer and it worked fine .I had installed Mplayer .Later i found out that i could watch the flv videos on the media player itself after install the "extra decoders support" so i uninstalled mplayer and installed the extra decoders ..but that did not work ..and the media player did not play the flv files saying "Unable to play media .Media format not supported" .After that even the regular avi,mp3,mp4 files that used to work stopped working and gave the same error message .The restart fixed it but as explained by someone earlier in the post .. it reccurs when i play a file format like flv or mp4 that wasn't supported before.. I even went to extent of reflashing the device with help of the NSU .. still no go .. Infact i thought that it would do a complete system reset . but it not only did not fix the problem for which i had done the reinstall but now i also had to reinstall all the applications that i had on it before .. A real pain in the ***. Somone suggested installing the subtitles support in extras-devel ..still nothing ..the damn thing just doesnt go away ...I did think about flashing the eMMC .. but was too confused with the instruction and honestly did not have the heart to mess with the OS manually and mess my brand new N900 ... If anyone out there finds a way to fix it ..PLS post it so i can stop pulling my hair out