by Ken Task.
Would help to know a few more things (more specific) concerning your troubled server: operating system (and any specs like memory), remotely hosted (if so, with whom), php settings (do you have the ability to increase php parameters (timeouts, memory usage), etc.
As painful as it might be, suggest turning on debugging and attempt the backup again only this time, do not include all 7 quizzes - include only one - and also exclude users. That might give somone more specific information to go on.
'spirit of sharing', Ken