This is kind-of a new one by me, but it's possible that you've got Epiphany set as the default web browser, and that's causing the problems.
In KDE, go to the wrench icon in the lower left of the screen. This will open a utilities menu. Go to KDE Components > File Associations and then under "Known Types" go to "text". Select .html, .xml, etc. and make sure that your preferred browser is at the top of the list. I hope this helps.