@leocg:
@palx:

Thank you both for your great help in solving the mystery, it's much appreciated. Cheers, and have a great remaining weekend.

@leocg: I just flushed my Firefox cache and manually deleted all remaining files, then started Firefox with a Youtube video and pressed the right arrow-key. Now I get a semi-transparent circle with the number 5 in the center, which is surrounded by an arrow. The image appears centered. This image also appears after disabling the 'Enhancer' plugin and with an empty cache. So it's a Youtube thing. Damn.

@palx:
Of course, the Firefox plugin is from the Firefox store, for Opera and Vivaldi I'm using the Chrome store version. The Chrome store plugins are each stored in their respective subfolder unpacked. So I could easily review the sources of the 'Enhancer' plugin, and I did not find any trails of the image there.

Currently, I'm reviewing the Opera sources. I will post a reply once I have an update on the issue.

@palx:
I forgot, I also have 'Enhancer for Youtube' installed for Vivaldi, there I don't get to see those images either.

@palx:

Thanks for your input. Well spotted, it is the 'Enhancer for Youtube' indeed. I suspected this plugin at first, too, so I disabled it and checked Youtube again, but the images would still appear. I have 'Enhancer for Youtube' installed in Firefox, too, and I don't get these images there.

@leocg:

The image I get to see when fast-forwarding/rewinding with the arrow-keys in Opera is the one I highlighted with a red circle in my screenshot (added below again). Fast-forwarding shows this image on the right, rewinding shows it on the left. There's no other message/text/whatever shown. This particular image does not show up in Vivaldi nor in Firefox, so I think the source is the Opera browser.

Screenshot: https://i.imgur.com/0LnMZlN.jpg

@leocg:

1. Startup browser (Opera, Vivaldi, Firefox, etc.)
2. Visit the URL "youtube.com"
3. Select any video to play
4. Press left/right arrow keys to advance/rewind the video
5. For external sites (e.g. a games news page) read any news post containing an embedded youtube-video, play it and advance/rewind the video with the arrow keys.

@leocg:
Just did a test run with the current release of the Vivaldi browser (Chromium). No "5 seconds" images appearing on Youtube.

That makes 2 votes against 1 that Opera is to blame for these images.

@leocg:
Yeah, probably it is from Youtube after all. I just checked some Youtube videos embedded on external sites. There the "5 seconds" images do not appear, but they do show up when viewed directly on Youtube. Yet there was one externally embedded video where the images also did appear.

It is strange though that in Firefox you don't get to see these images at all.

@leocg:
No, it isn't. For example, try with the Firefox browser, you don't get to see these images there.