Installing a multiple bloggs on one database

I have successful installed another wordpress blogg within the same database as another Blogg.
It runs up fine, but when I try to make any changes to the Blogg, ie. add a post or page, the error message ‘You do not have permission to access the requested file on this server.’ appears. Then I am locked out of the admin and the webpage. The original (first blogg) is unaffected.
I have checked the wp-config. php file and the info seems ok. I have been deleting the new bloggs folder and reuploading (ftp) to get it working again. Then I have the same trouble. What am I missing that is having file permission trouble?