I get the error in the title of this issue when I try to load ido-ubiquitous. Emacs version is GNU Emacs 23.4.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10) of 2012-04-12 on bitzer.hoetzel.info.
I am not super well versed in elisp, but M-x apropos completing-read knows about:
but doesn't know anything about completing-read-function.
Well, it looks like they just added that feature in Emacs 24. I guess that explains why I didn't use it when I first wrote ido-ubiquitous several years ago. You'll have to revert to an earlier version, which you can get from the github tags.
I guess I'll have to go back to the old advice-based method of doing things. I'll release an update shortly if I can.
Just released version 1.5 which should fix this.