Using Kicad 5.1.6 on Windows 10 /64.
I’m using this guide to import a symbol.
Step 2: Click on Preferences > Manage Symbol Libraries
results in Kicad (either Symbol Editor or Eeschema behave the same) losing focus and becoming non-responsive. Any click on any Kicad window results in an error beep and no other response. The only escape is the escape key, which returns control, but does not open the symbol library.
It acts like an invisible window has been opened. I’ve minimized the sizes of the Kicad windows and moved them to the edge in case the new windows was on the bottom, but nothing is appearing there.