If thunar is working, that is a good sign. I would uninstall dolphin and reinstall it, but it may not allow that, since it is part of the meta package. I don’t know the apt commands, but there should be a way to force a reinstall of dolphin with all its dependencies.
Well I tried reinstalling through Muon, the graphical program manager, and I’ve also tried “apt remove dolphin” then rebooting. Which seemed to work fine. And “apt install dolphin,” which worked but didn’t fix the problem. Just back where I started.
But that might not have reinstalled all the dependencies. I don’t know about that.
Thunar is a little weird. It has changed all my file associations to Windows utilities, so I keep clicking on files and wine pops up and then they open in notepad or something. Internet explorer is not my first choice for looking at jpegs. That’s something I can fix, it’s just weird.