No matter whether I select files or directories to be compared, the "Compare files" button stays greyed out.
This is a bug in the KDE Frameworks, not in Kompare itself. *** This bug has been marked as a duplicate of bug 390024 ***
As a temporary workaround after choosing files typing anything in either box (even a space) activates the button. Working on the proper solution for the bug in frameworks though.
Just pushed a fix for this to kompare in git. Fix will be included in 19.08 RC (but just missed beta tag by a couple of days).