Fortunately I was still logged in to my site in Firefox. One of my users reported that she cannot log in. I checked it under several other browsers and I couldn't log in.
Thank God that I was still logged in in Firefox and could turn off the plugin: htaccess Control. From then on I and other users could log in again.
There is an EXTREMELY SERIOUS PROBLEM with the new version of this plugin!!! However, I would use it in the future again as it is very handy.