There’s no specific answer to why it happens, but it primarily occurs due to the lack of codec support to play the particular file. When VLC player shows it, it means that the VLC player is unable to recognize the file format. UNDF is nothing but an error code which means the file you are trying to open has an undefined file format.

3.3 Download Combined Community Codec Pack.

3 How To Fix VLC Does Not Support UNDF Format?.

2 What Causes VLC Does Not Support UNDF Format Error?.

Unfortunately there is no way for you to fix this. VLC does not support the audio or video format 'undf'. I press F9 to choose to play with VLC player and i see title VideoCacheView.cfg I press F2 to play it opens the Videocacheview folder! The video finishes and I decide to run VideoCacheView I'm using Mozilla Firefox 18 and watch a Youtube video > Was it raw format when it says undf format? > I wonder if its the saving to webm from the webpage that makes it playable from the cache too? > VLC player does indeed play it from the cache now! > Having gone to the webpage again and watched some of the video again but not letting it finish downloading then saving it as a webm file elsewhere then Using VideoCacheView again to look in the cache again its appeared as a 20MB webm file. > It was a 70MB file in my cache yesterday but in unplayable undf format. Other videos I fail to play with VLC from the cache say nothing with no time length. > I do come across undf format every now and then, so it would have been good to play undf videos at times. > said 'try media player classic, better than vlc imo anyway plus it can use all sorts of codecs' > I had Googled it earlier and this webpage > You should have just googled for 'undf'. > On Saturday, Janu10:28:53 PM UTC, David H. Unfortunately there is no way for you to fix this.' 'VLC does not support the audio or video format 'undf'. I tried to play it on VLC media player but it said

I watched some of this video on this websiteīut it was a bit long so I thought I'd watch it later by taking the file from my Firefox cache.

How to play undf files