Hope this may help some having blank page problems. My original problem was the following:

1- any visitors, webmasters can traverse the site without any problem.
2- only registered users had no access to the home page. to be more specific, the main page was loading but as soon as they input their user name/pass, a blank page was loading (apart from that everything was still normal. in spite of the blank page, they actually registered and if they could write the full path of a page they wanted, they could access it, except home page).
3- after checking all previous problems to no avail, i noticed a few claimed their problem started after installing newbb module. When i disabled the moduled, everything started to work flawlessly.
4- There was only one block (recent topics) from newwbb on main page and after this one a system block was loading.
5- i simply changed the weight of the newbb module so that it would load the last and the problem disappeared.

hope it helps someone.
regards,
xyz