"Continue button" grayed out in "installation type" screen during 13.04 installation

I clicked "Back" and then "Continue" again, and now it works. Go figure...

[Update based on comments: If this doesn't work, try it again and/or wait 15 minutes.]

On Lubuntu 16.10, for me, not selecting "Download updates before install" solved the problem.

I also had this problem installing Kubuntu 16.10 in VirtualBox. Unlike shao.lo, for me dynamic vs. fixed .vdi didn't matter. For me the problem was that the virtual disk was too small, but the installer doesn't give any useful error report about this. There is an icon on the screen indicating the issue, but it's so abstracted as to be meaningless: it looks like a trashcan with a line through it, with no accompanying explanation. (If the disk is adequate it will be a checkbox.) I created a new VDI of appropriate size (>8.6GB, apparently) and it worked.

I was only able to discover this by running "ubiquity -d" from a terminal and examining the output in /var/log/installer/debug.