Without seeing your .rp file it is hard to say. The fact that it is “very complex” and “a couple years old” are two big hints, though --also reasons I probably would not want to look at it. But you can always send the file to email@example.com --they are usually pretty good at figuring out things.
Most browsers have “recently” (in the last year or so) changed the way they handle “automatic actions” such as you might have in your OnPageLoad event. For instance, i have older RP8 files with video links that used to autoplay but now do not, or chains of actions such as checking a global variable and firing an OnClick for some widget–used to work fine, but now works only after a user “touches” a page by clicking anything, scrolling, etc. If a widget is hidden or in a dynamic panel that is hidden, its interaction code cannot be called from an OnPageLoad action, etc.
When running your prototype, you can look at the Console panel to determine what gets called by OnPageLoad and if any subsequent or corresponding actions actually take place. If you have conditions, double-check that they are still valid and all local variables are still pointing to the correct widgets.
in general, my workarounds have been something along the lines of placing my “deprecated page load” interactions on a visible widget in a visible dynamic panel, then shrinking that dynamic panel until nothing inside it is visible in its viewport area (making it effectively invisible to the user), then calling “Fire OnClick” for that widget.