I've implemented user accounts during Event registration by enabling the profile's advanced settings to give the user the option to create a Joomla account.
When a user registers for an event, they are correctly offered the chance to sign-up or login:
If you would like to create an account on this site, check the box below and enter a Username and a password. If you already have an account please login before completing this form.
The 'login' word above is hotlinked to a page which allows the user to login and then CiviCRM should return the user back to the Event Registration page. It does not - after logging in, the user just gets a blank page.
The reason seems to be that the redirect page is malformed; it is:
when it should be:
The & concatenators in the URL are being incorrectly escaped.
I have reported this on StackExchange:
including my own answer that suggests that the root cause is the coding of the return link in this URL when taking a user to the login page:
Similar behaviour has also been reported in the Contributions process here:
Problem has been re-produced on the Joomla Demo site.