PGAdmin 4 1.5 desktop mode not starting on Windows 7 64 bit

Hello everybody,
I have installed the latest release of padmin 4 1.5, and the pgadmin does not start. When I try to start the pgadmin, I can see the splash-window, but nothing more. There is no activity shown in the taskmanager at the process, and the memory-usage also does not change any more.
This also happens, when I try to run the pgadmin with administrative privileges.
I am working in an enterprise environment with Windows 7 64 bit.
Enclosed there is the logifle with loglevel 10.
Has anybody an idea where to continue in order to solve the problem?

I have created a config_local.py with the following content:
SERVER_MODE = False
DEBUG = True
FILE_LOG_LEVEL = 10

Because we are using a proxy I have edited the
pgAdmin 4\v1\web\pgadmin\browser\__init__.py
file, and added the following lines before line 453 (otherwise I get an error-message in the logfile, this worked for version pgadmin 4, 1.3):

#DP, 15.03.2017: enabling proxy support
proxy = urlreq.ProxyHandler({'http':'http://<user>:<pwd>@<proxy>:8080', 'https':'https://<user>:<pwd>@<proxy>:8080'})
auth = urlreq.HTTPBasicAuthHandler()
opener = urlreq.build_opener(proxy, auth, urlreq.HTTPHandler)
urlreq.install_opener(opener)
#DP END

Thanks,
Didi