Buttons of some plugins: only text of values showing

I have the problem that the plugins "Frontend Edit Button" and "SimpleSearch" don't show any buttons but only the text: PLUGIN_EDIT_FRONTEND_BUTTON.BUTTON_TEXT and THEME_NUCLEAR.SEARCH.BUTTON or THEME_NUCLEAR.CLOSE (seems to be the values:
<input type="submit" class="search-submit" value="THEME_NUCLEARE.SEARCH.BUTTON">)

I removed the theme and the plugins and reinstalled it, also deleted the cache, without success. Grav runs locally under PHP7.3 in the path http://127.0.0.1/grav.

I migrated from 1.5.10 to 1.6.8 before (copying the directory 'user').

What am I doing wrong?

PS: Same problems with other themes. Also e.g. the plugin 'Map Marker Leaflet' doesn't work after migration (doesn't show any content at all). No idea..