just posting this as an FYI in case someone else has the same problems.
I *had* a problem with 1.7.4 install (aborted the upgrade to 1.8 ... whose idea was it to require php 5.3 instead of 5.2 anyways?).
I could access the base site but not the scp pages; which makes it really difficult to use osticket.
I finally managed to debug and hack the problem:
1 - in main.inc.php FIND and change the ini_set display errors to SHOW (ie change the 0 to 1) *** otherwise you just get the useless IIS 500 page ***
2 - load the pages and read the php error message
3 - hack the INCLUDE_DIR definition to be location aware: if (realpath(dirname(__FILE__)) != "" then use current definition of include_dir otherwise use '..'.ROOT_DIR.'include/'
4 - test access from home page and scp page
5 - turn OFF display errors.
like I said this is a total hack. No idea why realpath returns an empty string only for pages in the scp folder; and I ran out of time to try and figure it out.
(what really made this an interesting problem was php wincache not seeing the code changes in the php files; I had to turn that off too).