During the initial installation and later via YaST, the keyboard was set to
German (with "dead" keys).

Problem:
After a reboot or after logging out and logging in, entering ' and e causes the
character string 'e instead of ������. 
A workaround is possible with Compose ' and e, causes ������.

The effect occurs in all applications and also in the console.

Fixing the error in a running session:
Call YaST - Hardware - System Keyboard Layout :
    In the configuration of the system keyboard is shown as active:
        German (with "dead" keys)
    now selection of German in the selection list 
                and then again German (with "dead" keys).
    Exit with Apply
From then on, entering ' and e correctly leads to ������ 

But after logging out and logging in, the input is again without effect of the
dead keys.

The effect is reproducible on my three different computers with Leap 15.3
systems.

Information on one of the systems:
Operating System: openSUSE Leap 15.3
KDE Plasma Version: 5.18.6
KDE Frameworks Version: 5.76.0
Qt Version: 5.12.7
Kernel Version: 5.3.18-150300.59.43.1
OS Type: 64-bit
Processors: 6 ������ Intel������ Core��������� i5-9400 CPU @ 2.90GHz
Memory: 7.5 GiB
gfxcard: Intel UHD Graphics 630 (Desktop 9 Series)
graphic-system: X11