Re: blank page when permission denied
Please, Upgrade to Tiki5.x asap (latest stable branch, with security fixes) and once done, apply the Debug mode Enabled profile to your site:
See http://profiles.tiki.org/DebugON
This will enable the error reporting, to see whether there is some php or smarty error, which produce a blank screen due to some bug, or it's related to memory isues (you would see the error message in the screen, even if anonymous)
Cheers