Video Thumbs altering

All .webm videos do this? or just that exact one video? Please try another .webm video if possible to confirm it's videos with that extension doing this. Very strange issue.

Ok, tested .webm and is working fine using the below sample file. Was able to upload a custom thumbnail without issue.

www.webmfiles.org/demo-files/

It is doing this with all videos with the .webm extension.

I just tested with a MP4 video stored on the same server, and it seems to being the same thing...
The videos are connected through the link system, rather than uploaded.
Could this be the problem?

The video where the image was accepted was uploaded through the standard frontend method...
These files that do not work for image replacement have been linked.

I had no issue adding a custom thumbnail to a .webm video that was uploaded or linked. Please PM backend super administrator login credentials and will debug further as I don't know what more to suggest. You can try using either of the sample files in the link I provided as well.

Have reviewed your install and worked perfectly fine when testing the sample videos provided above. There is normal browser caching kicking in so if you change the thumbnail from 1 image to another you need to wait for browser cache to clear or refresh without cache (Ctrl + F5).

As for your white page it's just due to the page not being done loading from the looks of it (flashes white momentarily for me then loads in), which appears to be due to just performance issues with your sites (your site is throwing a lot of notices from 3rd party extensions). You also have something calling "index.php?option=com_ajax&plugin=inlinemodestatelistener&format=json&state=1" which is taking quite a long time to load (appears to be "Ajax - Inline Mode State listener").

I've also tested the 2 video files you provided in PM and they also let me supply a custom thumbnail perfectly fine.

Not sure what more to advise, sorry. Backend isn't showing the issue at all or the performance problems.

Interesting!
Thank you very much for help... at least I now know this is a local problem here.

I have tried different browsers etc... perhaps I'll attempt a different machine.