First of all I thank both of you very much for your responses.
@acrylian
Since I mentioned my gallery when registering I thought, that this would be enough to show the effected gallery's path. I did not pay attention to the fact, that my gallery's url isn't shown to logged in members. I just figured it now. So sorry for any inconvenience.
I hope that the theme "Version 1.4.6, 2014-05-26 10:08:24" is up to date.
Since I do just a very little blog for my wife and kids while working away from home and only to show them how it looks like where I work, I may miss an update, although trying be keep up with.
@fretzl
Special thanks to you! I did as you suggested and it worked right away. You saved my day and the days of my family :o). Last night I switched to the "basic" theme, already, just to get rid of the problem, but have not been very comfortable with this "kind of" solution. Well, everything is just fine now! Thanks again!
Merry Christmas to both of you and your families and y'all have a great New Year 2015!