I almost heard you typing!
I'm not exactly sure what is going wrong for you but since safemode will prevent scripts from accessing files above their location, I think, my guess is that some of the the files on the include list are outside of your client site's directories and so off limits.
One thing that suggests that is this error specifically,
If I were to guess that that is outside of your client directory, would I be correct because I think that is where you problem with safemode might be.
I guess it would help to know your directory structure, where the client scripts are, what scripts and where, if any, those scripts are trying to access etc.
One way or another, you defnitely need to get safemode working because it is the only way to prevent a given clien't scripts from doing possible damage outside of their client area.
[EDIT] There is this info at phpbb's site that is basically says that safemode should work although it does have a little additional info. May be useful to take a look. http://www.phpbb.com/support/documen...stall#safemode