Pages that time out are usually due to connection issues, occasionally browser issues if there's maybe a compatibility / script conflict (i.e. a script on the page won't load fully and/or properly, causing the page to time out or interfere with smooth running - used to happen all the time when I used Internet Explorer).
If other pages are loading fine the only things I can suggest is to either try logging directly into PayPal and locating the transaction manually (you can look only at 'sent' payments, and narrow the date range if there's a lot to go through), and see if opening the dispute works from there; and/or if possible, try a different browser. I recommend Chrome at the moment - I'm still experiencing some Javascript issues with Firefox even after a recent update. (That's assuming you're not using the app).
