[gentoo-user] Problem executing pgAdmin4

[Petric Frank]

Hello,

pgadmin3 is masked now. So i installed pgadmin4 (current stable verion 3.0). 
Installed w/o problems.

But when executing it (in a terminal window) i get a dialog box saying
  An error occurred initialising the application server:
  An error occurred initialising the application server:
  %1

and the terminal window is filled pagewise with
----------------------- cut ---------------------------
Python path:  "/usr/lib64/python3.6/site-packages" 
Python Home:  ""
Webapp path:  "/usr/share/pgadmin4/web/pgAdmin4.py"
"Failed to launch the application server, server thread exiting.
\u0000➑\u0000\u0000\u0000榠\u1C93翱\u0000먀ᰆ翱
\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u000F\u0000\u0000\u0000＄
\u09FF－＆Ͽ\uFFFF\u0005\u0A0E＋（\u07FE\uFFFF\uFFFF＂\uFFFF，攍兇须꾭懰䀌翱\u0000\uEAB0ᜊ翱
\u0000㙔\uE4C8帚꿧\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000盚饇䥟璊Ꙑ䀌翱
\u0000\uA8C8ᜉ翱\u0000鯩슄鼗䷷\u0E70䀋翱\u0000偈䀌翱\u0000✠徰⼬蘉좸⠪翱\u0000ꃘᜋ翱\u0000ⷬ행膀ꈋꈨ䀌翱
\u0000垀ᜋ翱\u0000沦⻋䎛緒짐⠪翱\u0000嚠ᜋ翱\u0000段ꉀꒅ텣遰⠤翱\u0000撈ᜋ翱\u0000⻲幕炈멕胰䀌翱\u0000攐ᜋ翱
\u0000\uE6A3ᓐ溪껊쫨⠪翱\u0000斘ᜋ翱\u0000\uF42F끒䙭㔋昰䀌翱\u0000映ᜋ翱\u0000盰\uE66A⍷캪限ᮑ翱\u0000
좸ᜊ翱\u0000뱾ס⏤鱑룰⠁翱\u0000춸ᜊ翱\u0000ꢦ蔞蠹꾞㨈䀐翱\u0000츈ᜊ翱\u0000\uF20E㥑䶫텓움⡌翱\u0000饠\u1CBB
...
----------------------- cut ---------------------------

Selecting "Ok " in the dialog box i get an other tabbed dialog box asking for 
a browser command (currently empty) and Python paths (preset values seem ok).

Entering "/usr/bin/firefox %URL%" as browser command does not change anything.

Any hints ?

Info:
  Kernel 4.14.65 (Gentoo)
  KDE Plasma (kde-plasma/plasma-meta-5.13.5)
  Memory 16 G
  Apache installed but not running
  Browsers konqueror and firefox available
  pgadmin3 still installed
  Python 2.7, 3.6 and 3.5 installed (= "eselect python list" output)

regards
  Petric