I am not sure this is related to the admin login problem,
I also have a problem when I try to change the tinymce plugin
If I try to allocate default zenphoto editor to albums ( as zenpage had been accidentally selected) I get a
ERROR 500 - INTERNAL SERVER ERROR. I have checked with my web host and they think permissions are correct. 755 on folders, 644 on all files except a few in zp data have 600.
The only error I can see during setup.php is debug is enabled but I am not sure where I can disable this.