It's been a while since I've played DVDs on my computer, but have a look at this page:
https://help.ubuntu.com/community/RestrictedFormats/PlayingDVDs
Looks like you might need to install libdvd-pkg
sudo apt update & sudo apt install libdvd-pkg
There was a theme that was suppose to replaced the reworked Ambience, but I guess it never took off. I think it was called Yuyo, and was introduced in 15.04.
Try doing a web search for "kde ubuntu raspberry pi". I did that and found this.
https://manjaro.org/downloads/arm/raspberry-pi-4/arm8-raspberry-pi-4-kde-plasma/
This is probably a bit to specialized for this subreddit. You could try opening an issue with caja-dropbox, or even compare the closed issues to see if you get a hint.
Having a look through what has been done with the underlying issues with Python-caja this should now be fixed.
https://github.com/mate-desktop/python-caja/issues/22
https://github.com/mate-desktop/caja/commit/8a826a675862d56b6d01c8467282bde9171f8cf8
I'm running 16.04 since the first Beta, I have not run into this issue. I see no reason why this issue should not be resolved once you are running 16.04 as the fixes are now in place.
You can wait the few days until 16.04 is released or force the upgrade now to test it out.