It look to me that too few have this problem (only you ?) and it may be something about your configuration or environment.
It is unusual you can’t see anything in your log (from the server, phperror, etc).
Did you check that ?
Also, I would try with a brand new database (keep the first one aside) to test if registration work with little change to default setting. Then I will configure option by option your clean Tiki to use the same setting than your real Tiki. This to try to see what is causing the failure.