looks like a server error was thrown, that caused this JS error.
in DB, in the stored SP 'yaf_forum_move',
the line : 'exec [dbo].[yaf_topic_move] @tmpTopicID,@ForumNewID,0, -1;'
needed another argument which is the '@UTCTimeStamp'.
i set that argument to optionnal, and now it seems to be working.
And also, in yetAnotherForum/pages/admin/deleteforum.ascx.cs, around line 290,
there is a '// TODO : Handle Error Message?!' ;)