Inkscape crashes when opening or importing a file or when trying to save a file.

open Inkscape
try to load a file (Ctrl-O): Inkscape crashes immediately (Program disappears from screen, Inkscape is no longer visible in the table of processes of KSysGuard, the system monitor.)
open Inkscape
try to import a file (Ctrl-I): Inkscape crashes immediately.
open Inkscape
do some drawing
Try to save the drawing (Ctrl-S): Inkscape crashes immediately.
open Inkscape
do some drawing
try to save drawing as (Shft-Ctrl-S): Inkscape crashes immediately.
open file manager
double click on an existing .svg-file
Inkscape opens normally
do some drawing
save the modified file: - no problem
try to save the modified file as (Shft-Ctrl-S): Inkscape crashes immediately.