I am testing the registration procedure for new members

I am testing the registration procedure for new members using the Membership plugin.

Here's the screen shot illustrating the problem:
http://screencast.com/t/RrEnrf59C

In short: trying to register results in the message "Problem with registration".

Since the error message does not state what the problem might be or what to do about it, we can only assume that for some reason something somewhere in the uncertain world is somehow preventing the registration from going through. Somewhat.

Any advice?