I face same issue with my Dropbox client after upgrade my system (Manjaro KDE 18.0.4). After finding on Google I see that it already reported as here: https://www.dropboxforum.com/t5/Installation-and-desktop-app/Trobles-with-opening-site-from-app-in-KDE/td-p/270031
I have Debian 9.4 with KDE 16.04.
When I try open dropbox site, from right-click menu on tray icon or from context menu in dolphine, system open html file (‘/tmp/xxxxx.html’) in Kate.
System think, that this file is Perl-script.
Due to marked as solved, but I cannot find any solutions to fix it in my case on the Dropboxforum:
Since you’re running Debian, currently this isn’t a supported OS for the Dropbox desktop application, as only Ubuntu and Fedora are the Linux distros you can officially use it on.
So, I find a way to fix this issue that just set the OS open the Perl-script file (that is generated automatically by the Dropbox app) by browser app (Google Chrome or Firefox, …). Follow steps on my video in below for more details.