Thank you for your answer!! Meanwhile I tried another way (the magic device tool) and it worked just fine. Still don't know why the installer didn't work though..
But thank you anyway!

Hello, I am trying to do exactly the same thing: installing Ubuntu Touch on a Nexus 5 32GB. But I think the Installer really got stuck. It says "Installing. Installing Ubuntu Touch on your device", and further down: "Waiting for device to enter recovery mode... Flashing recovery and boot images". On the phone I see the recovery screen (with the options mentioned by Andirs26). I did nothing but now I have been waiting for over 30 minutes and nothing has happened so far...
What could I do?