well, may be a coincidence or a clue, but if you submit that form without having selected any project - you'll get that error.

should there be error checking and a friendly response in this case? IE: Your attempting to add a user to a project but have left a field blank.

I should add that I wasn't able to reproduce this problem (aside from not selecting a project) in the current CVS (feb 25th)

Uhm, I just checked (out) and it's still there for me. And I'm even sure I'm selecting an unassigned project from the list.
From what I'm seeing in mysql.log, Mantis is generating the right query

SELECT *
FROM mantis_project_table
WHERE id='15'

where 15 is the existing id of the project I'm trying to assign. Strangely around line 62 that returns a zero-length result...

Fixed in CVS