How to debug what module/process is requesting a file? I moved my drupal 6 site to server with php 5.5 and the site started to throw not found error -  sites/all/libraries/jquery.ui/themes/base/jquery-ui.css on every page. I have not installed any new module and the file was never on that location. How would be possible to debug this? Thank you.
It is possible that the previous server was suppressing errors entirely and this error would have shown up had error reporting been turned on. In general, you can debug such errors by using the dsm() function from devel module. If you need to know where a functional call is coming from, you can use debug_backtrace().