Ok, I got it. It’s hard to say, what I did exactly but something like this:
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First I used Warehouse to uninstall some GStreamer-Plugin-* stuff. That resulted in jolla-camera being uninstalled.
So I went to ssh and reinstalled the packages: zypper install jolla-camera-lockscreen jolla-camera qt5-qtmultimedia-plugin-mediaservice-gstcamerabin. That brought camera app back, but did not make it work. -
Then I used zypper to uninstall some gstreamer/codec stuff from repos openrepos-lpr and openrepos-lpr_A7. Among them was: libout123-0, libwebp6, gstreamer1.0-plugins-bad, libvpx5. Zypper did not just remove stuff in this process but also pulled in replacements for some packages. That resulted in YouTube working!
Seeking does not work. (I am pretty sure, at first seeking worked until I fixed the camera, like below, but I may be wrong. It’s late.) -
Then, I did pkcon install gstreamer1.0-droid. I don’t know, if I should/could use zypper instead, but anyway: Camera started working!
Both of them and flashlight.
Sorry for my hardly reproduceable chaotic “method”.