Base URL - Error Message

Hello! Sorry for my bad english.
I was going to change the "Base URL" because it was set to my homecomputers (localhost something). When I type in what I think is the right and click on "Save changes this code comes up:
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Not Acceptable
An appropriate representation of the requested resource /admin_options.php could not be found on this server.

Apache/1.3.34 Server at yyyyy.xxxxx.se Port 80
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It works correctly for the user but as administrator I can't erase a thread or change in the admin because this comes up then:
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Felaktig HTTP_REFERER. Du refererades till denna sida från en icke-auktoriserad källa. Om problemet kvarstår, kontrollera så att 'Base URL' i Admin/Options är korrekt och att du besöker forumet genom att navigera till denna URL. Mer information finns i dokumentationen till PunBB.
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The problem as I see it is that I can't see what Base URL I schould use.