My b2evolution Version: Not Entered

Hello everyone,

I'm having a strange problem as follows: I can do whatever I like in my (freshly installed) b2evolution (3.3.3) - create entries, delete them, upload pictures, create and manage categories, you name it.

BUT I can't edit entries. Every time I try to edit a page I'm getting a "403: You don't have permission to access /admin.php on this server.".
Which I don't get at all. If there really was a problem with file permissions I guess the problem would always be there, not just on that specific action, wouldn't it?
(everything's chmod 777 and there's no .htaccess file)

Does anyone have a idea about that? By the way, I could edit entries before. It kinda seemed to develop that problem spontanously after a few edits!?

Any help is greatly appreciated.
Thanks.