My guess is it has to do with the text editor, probably not getting the js loaded that is needed for it, I switched your comments to use the regular textarea form and it is working now.
So plugins is a profile, except there isn't a user associated with the profile, is the site master admin suppose to be the user associated with the plugins profile?
izhmel:
The site was not crated by site builder, how do I show the contents of the plugins.tpl file?
plugins.tpl is a custom template that would have been created by you, or maybe it is a profile, that is the only two things it can be. Is that the only page that is white screened? If so, login to the ACP and look for a profile named plugins.
If this is not the case, can you send you admin login so I can check it out?