HELP, plugins and themes gone!

I have upgraded to 2.3 (from 2.2.2)

Everything works perfect, BUT;

in admin "backend"

Plugins
and
Themes

choices from the top /Gallery is not selectable, I get a blank webpage!

http://itrollheimen.net/Gallery/main.php?g2_view=core.SiteAdmin&g2_subView=core.AdminPlugins

I have done an error, I have chosen to get quadratic thumbs by a mistake, I cannot longer correct this.

Please help me, if I cant get back the plugins themes, how do I hack the gallery 2.3 to get back my normal thumbs?

Thanks so much
www.itrollheimen.net