Video Thumbs altering

Hi there...

Joomla! 3.6.5
Linux
PHP 5.6
CB 2.1.0
Gallery: 2.0.0+build.2016.12.15.00.47.58.a60558548

Small issue going on here...
When a video is uploaded, the automatic Thumbnail is determined from an early stage in the clip.
Often from the first image of the clip.
So, to alter this I want to be able to upload my own Thumbnail... and this is available once re-entering the video for Edit.

However, when I try to upload an image file for this, the page breaks to the white page of horror...
This also happens if I try to just give a link for the Thumbnail.
Any tips?

I have tried from different browsers.
I turned the fault showing to maximum, and that didn't give any clues either.

White pages are usually a fatal error of some sort. Often a memory issue. Enable both debug mode and maximum error reporting to expose the error. If still not displaying check server error log. Ensure you've latest CB and CB Gallery installed. The thumbnail for youtube videos is provided by youtube it self so it's accurate to whatever thumbnail youtube is actually set to use (which I believe you can change on youtube if it's your video).

Everything is as updated as possible.
Again I turned on the debug mode and maximum error reporting, and still I get a really clean white page. No messages of any faults.
I checked the server for faults, and as you see from the screengrab (see top image), there are no clues there either.

The memory is not an issue, as I have 32 MB covered with 64 MB to post.

Yes, I know that video sites such as Youtube etc have predetermined video images.
My perspective is more that video upload possibilities etc are for videos that are not on those sites. I mean, I don't really see the real point of building a site, just to repost Youtube stuff, other than perhaps one here and there.

I don't want to post to Facebook either, due to Copyright issues etc.

But, this is not a huge issue, as everything is working, the vids are playing etc.
The image that is getting placed automatically does work. So this replacement of image to a chosen one is more a wish, since it seems it should be possible.

Try logging out and back in again to purge the gallery parameter cache then retry. I was able to upload a custom thumbnail for a youtube video so I'm not sure what to make of your issue as you've no errors. What is the URL (with domain removed) of the white page?

The white page has the URL defaulting to SITE/administrator/index.php (I gave up trying frontend).
So, not much help there...

Prior to this, the URL is:
SITE/administrator/index.php?option=com_comprofiler&view=editPlugin&cid=529&table=itembrowser&action=editrow&mediaid=1600&cbprevstate=b3B0aW9uPWNvbV9jb21wcm9maWxlciZ2aWV3PWVkaXRQbHVnaW4mcGx1Z2luaWQ9NTI5JmFjdGlvbj1zaG93aXRlbXM%3D

-which is as it should be...

Yes, altering the image for a Youtube video is not a problem.
Even altering an uploaded video image where the video format is .MP4 also works. (Prooving this is not a cache problem etc).
The video format being used on these particular videos is .WEBM
Not sure if that has any "issues" in this regard, but it appears this may be the culprit?

Attempted frontend altering again, and the same respnse there still.
No luck...