Some mp4 files taken by my cellphone (haier w910 using bravo camera) stopped
playing in my pc, while others do play. I tried with several programs such as vlc, windows mp, classical mp, quicktime, sony vegas.

I have checked with mediainfo and noticed that the bad ones (that do not play) has a tag:

Xtra : WM/MediaClassPrimaryID

while the good ones do not have this tag.

I could not find over the net enough information about such bug, unfortunately.

Can someone help? Is there a recommended way to remove such tag?

I am almost sure that I checked these files earlier in the pc. Is this possible that some software added this tag?