Windows – Suddenly unable to play mp4 videos on Windows 10

In the event someone has a similar issue ...

As it turns out the problem was related to digital certificates on the server(s).

On the production box, the certificate name matches the DNS name. On the test boxes, it did not. (The test servers each host a number of test web sites.) So you'd get a certificate ("name mismatch") warning. BUT after dismissing the warning you could still download static content of other types -- we have PDFs and images (in various formats) that all worked fine -- it was just Media Player that, for whatever reason, wouldn't work.

Our server admins had assured me that once you dismissed the certificate warning, the certificate name mismatch should no longer be a problem. And that seemed reasonable to me and in line with what I knew, otherwise I'd have mentioned that in my original post.

But when I tried accessing the MP4 file with Fiddler running, I got a second certificate mismatch error popup (from Fiddler) ... and then after I said "ignore this error and continue" the video did in fact play.

So my best guess is:

• the Media Player download request was failing silently because of the name mismatch (instead of popping up another warning dialog)
• ... resulting in invalid content
• ... resulting in the error message implying there was a codec issue.

It seems to me that several things went wrong here.

• The certificate should not have been a problem, since I had already instructed the browser (IE as it happens) to ignore the "error".
• Even if it was a problem, I don't understand why I didn't just get another certificate warning (until I tried hitting the site using Fiddler).
• If the error had been trapped correctly within WMP, I should have gotten a different (less misleading) error message.

But the certificate was about due for renewal ... so when it was renewed we had the site name added as an "alias" (Subject Alternative Name aka "SAN") on the cert. So we no longer get the certificate name mismatch warnings on the test site(s) ... and the video plays just fine.

This does seem a comprehensive file format list, but you can check the file headers to make sure what the format actually is, as opposed to simply change the file extension that may confuse whatever player you are trying to use.

http://en.wikipedia.org/wiki/List_of_file_signatures

Next, make sure you install a video and audio codec pack so your video player can understand the coding used on the file. Something like @Santeador suggested. I personally have used http://free-codecs.com/K_Lite_Mega_Codec_Pack_download.htm

Another thing to check is that the file size is consistent with the information on Google Play, maybe the download got interrupted and you need download the file again.