I have cloned menu id 42 and I can edit this in all the relevant ways but when i hit the "Save Options" button I get the message ". 403 error" I think the . is actually a bullet point,
and the changes are not saved.
Error 403 means "forbidden" and the save function checks the permission
tiki_p_edit_menu_option, which presumably you have to be able to load the page at all... so i don't understand why this should be related to the size of the menu.
Do you see anything related in the apache error_log?
Maybe raise a bug report set up a show instance on dev.tiki.org?