I followed the tutorial on MultiForm in The Complete Guide ... and got stuck on starting the MultiForm using the RegistrationPage. Foud out that if I left the "extends MultiForm" out in the RegistrationMultiForm class (so no inheritance), made my own constructor and put a debug in it, that debug showed up. If I reactivated the extends MultiForm I got that blank page again.
I replaced the 0.3 MultiForm with the 0.2 MultiForm found in the book code examples, did a dev/build?flush=all, and it worked right away.
There must be some error in the 0.3 module that either prevents it from installing completely a/o correctly, or installation of 0.3 was all right but there is some condition in the classes that cause this error.