In "Preferences" > "Interface" > "Windows", I am trying to activate "Save and restore windows geometry for each document".
But Inkscape simply refuses to do this. Yes, I did a restart to active this modification.
What am I doing wrong or what did I forgot to do?
I am using version 0.91 and running PCLinuxOS FullMonty 64 bit as O.S..

It's hard to say, without some additional info. Can you tell us about an example of the problem? Meanwhile, I'll check bug reports...

Ok, I found a couple of bug reports:

https://bugs.launchpad.net/inkscape/+bug/195683
https://bugs.launchpad.net/inkscape/+bug/1530582

Do either of those match your circumstances?

brynn, sorry for my late response. A lot of other issues took my attention.
But my problem resembles closely that of bug number 195683.
I am afraid I have to wait until this bug is solved , or can I do some fiddling?

You could ask on the bug report if there's anything you can do to help solve the issue.
Most probably it will be tackled when Inkscape moves to Gtk3 for the next version.

This problem seems to be solved in version 0.92, but you never can be for sure...