First of all, I don't know whether I should send bug reports to this list, or put them on Bugzilla, or both... is there a recommended protocol?

Anyway, there's a bug in the handling of properties of type "Page" that are enumerations - fields that have this property get a "not on the list of possible values" error on the screen if they have a value other than the very first one on the list. I traced this problem, and found the issue: a variable is created, in "SMW_DataValue.php", to loop through the list of allowed values, checking each one against the current value. In the case of a wiki page, however, that variable keeps a cached version of its title, that is set the very first time and then is never reset. There are are various ways to solve this problem, but the easiest might be to change the function parseUserValue() in the file "/includes/SMW_DV_WikiPage.php". You can add a new line after line 26, so instead of

                        $this->m_value = $value;

...it now reads

                        $this->m_value = $value;
                        $this->m_title = null;

This will prevent the old value of "m_title" from getting read and used in place of the actual value.