Error "The process for the file protocol died unexpectedly." in KDE 5 apps which use KDE 4 libraries

[Kai Herlemann]

Hi all,

I get the error "The process for the file protocol died unexpectedly."
if I press, for example, File/Open" or "File/Save in Okular (version
0.26.1). All apps with that error message are using KDE development
platform 4.14.26 (Information of Help/About <programname>, the
dependencies and "lsof -p PID").
I'm wondering about that, because the concerned programs are KDE 5
programs and as far as I know, KDE 4 libraries and programs are
discontinued.

I got that error message at the first time after a upgrade (I'm using
Debian testing). I tried to solve it by moving
/var/tmp/kdecache-<username> and some KDE files files in  ~/.cache to
backup folders, but it didn't helped.
How to solve that problem?

Thank you in advance,
Kai