As explained in a previous post, lots of updates occured especially dealing with rpm. And here we go. We are facing at the moment some nasty issues in installer. The graphical part of the installer is just crashing due to a bug implying glibc and rpm. Both were recently updated.
Work is in progress and the issue was reported upstream. You can follow it here on the bug report. As soon as this bug is fixed, we will be able to release the Mageia 5 beta 1 isos. Stay tuned!