Hi there,
Please also be aware that there are ~35,000 undergrads at USyd, possibly many of them trying to access their timetable as well as all the unhappy vegemites that were lining up outside Madsen to have their timetable changed so the server is just a -tad- busy. This time of year is always the most hectic, so be patient and eventually all will be well.
If the browser is giving you the "your browser sent a bad resquest" message, then it is possible that your browser is at fault, in which case, try a different browser/computer.
The final option is to go into uni, and you should get out there for the clubs and socs anyways!!