could not find or load the Qt platform plugin "windows" - PyPlot, Julia 0.6.1, windows 10

Hello,
It seems like there were a lot of related issues in the past, while I had never encountered the problem before: it seems that no backend is available for pyplot any longer.
I get this error message:
"R6034 An application has made an attempt to load the C runtime library incorrectly...."
Then I click 'OK', and it gives:
"The application failed to start because it could not find or load the QT platform plugin "windows" in "",
Available platform plugins are: minimal, offscreen, windows...."

I tried to change the backend by setting:
ENV["MPLBACKEND"]="tk" or "qt4agg" as it was suggested elsewhere, but it did not change anything.
I'm using the default python distribution from conda.jl obtained from PyPlot.

I've tried to uninstall and reinstall julia and packages without any success.
(Actually I had the problem with julia 0.6.0, then moved to 0.6.1 without it solving the issue.)

It's a recent problem for me, it used to work a couple of weeks ago.

I would be grateful if someone could help.

Best regards,

Dim.