I don't useing .httaess in this site.are you useing .httaess or something like that?
I Try two times install system. The result is the same. No plugins is installed, clear wordpress version from wordpress.orgWell, you can try tweaking it but what I suggest is to have a fresh installation to prevent any future bugs.
Try deleting .htaccess files from root directory of your forum and wp-admin too.
What plugins have you recently installed ?