What has changed from the last time you had success using Xn shell extension?
I've no idea. I did not recognize immediately, but then I found that the XnView Shell menu disappeared from the explorer.
Are you sure you're using x64 version on x64 OS?
Yes. I'm using x64 OS. Installing a x86 DLL is not working. Most of my applications are x64.
Do you have any advanced security software like ZoneAlarm working which can potentially silently block your register attempts?
At the beginning I thought it was the antivirus, so first I tried to deactivate, then to remove it... without any results. I do not use any other security program.