Hello !
Another Udate to the behaviour described below: I updated KiCad to V5.1.9 on two machines and this effect did NOT appear ! Everything was just fine. Seems I will never know what was wrong…
Yesterday I updated my KiCad 5.1.6 to 5.1.7 on Linux Mint 19.3
I added the new ppa using the command line and then installed the whole list of updates for Mint.
Then I made a RESTART (old windows habit 
)
I startet KiCad and the Main window appeared.
I tried to open my last project but as soon as I chose EEschema or PCBNew the window disappeared and nothing else.
(I didnt try the terminal output, sorry)
OK, maybe something wrong with old path, permissions …
So I repeated this with a new 5.1.7 project. Same reaction
Then I did what ? RESTART the system a second time…
From now on everything was just fine: new project, old projects - no problems.
By the way: thanks for the “save as” option 
Just an Update: This happens also after the 5.1.8 Update…