The plural map contains:

// babies (baby)
array('sei', 3, false, true, 'y'),

And as a result, movies is singularified to movy. Symfony is now looking for 'addMovy' and 'removeMovy' instead of 'addMovie' and 'removeMovie'.

(Is there a possibility to hint for function names by the way?)

The word 'movies' seems to be an exception to the general rule. I checked a couple of word sites and there are only 2 words ending in 'ovies': movies and anchovies. The singular of the latter is anchovy. All other words ending in "vies" should be singularified to "vy", as far as I can tell. This would result in an exception for just the word 'movies'.

Putting this in front of the 'babies' entry solves the issue:

// movies (movie)
array('seivom', 6, true, true, 'movie'),

Added a pull request, my very very first pull request on GitHub. Hope I did everything right.