What are good causes to reopening a judgement

The rules to reopen or set aside a judgment vary from each jurisdiction and it depends on how judgment was obtained.

If you are talking about setting aside (or vacating) a default judgment there is generally a three part test as follows:

1. The defendant must explain the default (why he/she did not respond)

2. The defendant must act in a timely fashion upon becoming aware of the default.

3. The defendant must have a defence on the merits.

If the judgment was obtained at a hearing, in particular a trial, then a motion for a new trial could be brought if there was a mathematical error in the judgment or if there was new evidence that could not have been reasonably available to the defendant at the time of trial.

Otherwise, and basing it on the rules of procedure where we are located, the only other way to reopen a judgment is by way of an appeal to a higher court. To succeed there generally must be an error of law.

Many people argue no service or inadequate/improper service of the original Summons and Complaint. Not necessarily a winning argument but it is an argument.