A big problem after updating

After updating K-Lite, thumbnails stopped to work for some videos. I think it is those which worked earlier due to installing K-Lite. After I tried to fix it, system thumbnails stopped to work for mp4 and avi. But it works for mov and m4v. And if I rename mp4 to m4v, a system thumbnail appears.
Reinstalling/deleting K-Lite doesn't help. Also tried resetting settings and tweaks.
System - Windows 7. In Safe Mode, all the same. So probably something wrong with the registry (I hadn't modified it myself before it broke).
Windows Media Player works for files without thumbnails.
I need to fix system thumbnails at least for mp4. Please help.

Uninstall K-Lite and also Icaros if you installed that manually as well. Then do fresh install of only K-Lite.

Then check contents of registry key. It should look like this. If it differs you might have issues with registry access permissions.

Code:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.mp4\shellex\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5aec3ec-e812-4677-a9a7-4fee1f9aa000}"

[HKEY_CLASSES_ROOT\.mp4\shellex\{E357FCCD-A995-4576-B01F-234630154E96}]
@="{c5aec3ec-e812-4677-a9a7-4fee1f9aa000}"


Yes, I had both installed when it broke.

Uninstall K-Lite and also Icaros if you installed that manually as well. Then do fresh install of only K-Lite.

I already tried that.
I have these registry names and values when Icaros is enabled (but the second name is lower case). Icaros works (and worked), but I need Windows thumbnailizer for mp4 (because Icaros displays different images, it doesn't know the Windows algorithm). So the problem is that Windows thumbnailizer stopped to work for several formats. But it still works for mov, m4v, wmv. Even if K-Lite and Icaros are uninstalled. Appearing a system thumbnail after I rename mp4 to m4v is really weird.

It did not stop working. It is using Icaros instead. It is important to be clear when explaining a problem.

You can adjust the algorithm for Icaros. By default it takes thumb at 25% of video duration. You can adjust that % or use a fixed time offset.
Codec Tweak Tool > Thumbnails > Options

But what is wrong with thumb chosen by Icaros?

For system thumbnailer for MP4 replace {c5aec3ec-e812-4677-a9a7-4fee1f9aa000} with {9DBD2C50-62AD-11D0-B806-00C04FD706EC}

I meant Windows thumbnailizer stopped working ("system thumbnails").
I can't make the same images with Icaros. It looks like the Windows algorithm is no so easy like just %. Even if I adjust it for 1 video, others have new thumbnails (not which were earlier). So I didn't recognize videos by images anymore. I found that Windows mostly uses key frames.
I have {9DBD2C50-62AD-11D0-B806-00C04FD706EC} when Icaros is disabled, and no thumbnails for mp4 and avi.
Using Microsoft thumbnailizer for mp4 is critical for me.

{9DBD2C50-62AD-11D0-B806-00C04FD706EC} is the Microsoft thumbnailer

Yes, I know.
But how could it stop to work for mp4?
It is only because of K-Lite or/and Icaros. I only worked with them when it broke.
What can I try/check?

Neither app break the MS thumbnailer. The only thing that they change is that registry key.

Try clearing the thumbnail cache.

I have no idea what is wrong with your system.

I tried various buttons in K-Lite to fix that several videos are without thumbnails before I noticed that all mp4 and avi stopped to work - so it is not only those 2 registry keys. But I didn't go in very deep settings.
But resetting settings doesn't return it.
Creating a new user and deleting the cache folder didn't help.

Did you do stuff like disable Media Foundation or any of the Microsoft codecs? Re-enable those.

I could do this. Shouldn't reset return them? I did reset several times.
But codecs seem to be ok because it appears if to rename to m4v.

But you have set m4v to Icaros? Icaros uses its own codecs and is fully independent from the MS codecs.

No, I haven't. m4v is 100% Microsoft. Because if Icaros is enabled for mp4 and I rename an mp4 file to m4v, thumbnail changes, and I see that it becomes the image that was before the break. And m4v works even when K-Lite and Icaros are uninstalled.

Maybe Process Monitor can reveal what Explorer is doing differently for mp4 and m4v.
https://learn.microsoft.com/en-us/sysinternals/downloads/procmon

Thank you!
I have already tried procmon, but I didn't understand how these extensions keys work then and was skeptical because Explorer does very many things in the registry at renaming, I had to try one more time.
This time I saw that it reads HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\PropertySystem\PropertyHandlers\.m4v
And .mp4 had a different value. And many other extensions.
It started to work. Unbelievable.
One of them incorrectly modified the registry (a bug). It was: {0C08E3BB-D10B-4CC9-B1B3-701F5BE9D6EC}. Google finds Icaros. And I remember that it was after pressing in Icaros (I wasn't sure). Damn beta version. Spent a whole day for fixing it with AI and Google. And he even didn't want to reply after I described the problem, I waited 2 days and went here.

I returned to the initial problem (which was before the break): some videos are without thumbnails (even some mp4). After installing K-Lite several years ago, videos without thumbnails got them. Without changing displayed images (Icaros change). As I remember, I did nothing, just installed. And all videos were with thumbnails. But now even installing K-Lite Mega changes nothing. Why? What to do?

Windows sometimes may use the MS PropertyHandler for thumbs. Xanashi should know more about it.

K-Lite doesn't modify the propertyhandler for mp4. It only uses the Icaros propertyhandler for mkv and formats that aren't handled by Windows.

The Icaros settings tool allow you to choose for which extensions to enable it when you use a manual installation of Icaros. It is not a beta issue.

The MS thumbnailer uses image from beginning of video. Setting Icaros to 0% should give similar result.

I updated Icaros to beta, and it left its keys in PropertyHandlers after I deactivated it, that is why the MS thumbnailer stopped to work.
It looks like I found a good % for Icaros.
But those codecs which the MS thumbnailer uses to get frames, can they be replaced with more powerful/new ones to work for more files and maybe faster? Just interesting.

No, you can use either MS or Icaros. The MS handler only uses media foundation. There is no alternative codec in that case.