I am opening this back up because there is a conflict with the recent release of Joomla 1.5.25's upgrade from 1.5.23. Apparently, this upgrade will result in DTR page, Next Page, to freeze after confirming registration. I know this because I downgraded the site from 1.5.26 to 1.5.23 and that restored functionality to DTR.
Somewhere in the upgrade files there is a file causing this conflict.
Posted on Joomla.org to see if there is an answer. Here are the upgrade packages as zip files. There aren't many and I believe the issue is in the upgrade to from 1.5.23 to 1.5.24 or then to 1.5.25.
I have attached their upgrade files here if needed.
[attachment=1:1qvrv4h9]<!-- ia1 -->Joomla_1.5.23_to_1.5.24-Stable-Patch_Package.zip<!-- ia1 -->[/attachment:1qvrv4h9]
This is happening to us also. We found out that it seems like Internet Explorer doesn't like something on that page and will not allow people to click next. But in Firefox or Chrome people haven't had any problems clicking next.