I registered on the site, choosing a username and password. I received an e-mail with an activation link, which I clicked on.

So then I went to log in for the first time, so entered my username and password, and kept being told the password was wrong, so I requested a reminder...

And it turned out my password was a (clearly system-generated) random letter/number combination password which a) I hadn't chosen and b) I hadn't been told about and could never have known about until I requested the password 'reminder'.

Same here and there's no option to change your password