Steps to reproduce the issue

• create multilingual site. - have multilingual articles and assign them with other articles in the article edtiting tab "association". - edit the article in frontend. - click save - get warning message "Warning: Field required: Category" - article will not be saved

Expected result

Should be able to save the article

Actual result

article will not (and can't) be saved because of the warning message "Warning: Field required: Category"

System information (as much as possible)

CMS 3.3.3
PHP 5.3.x and 5.4.x

Additional comments

Since updating to J3.3.3 I cannot save articles in the frontend, that have association to articles in other languages.
If I edit an article with assignments in frontend, after clicking the save button I get the message "Warning: Field required: Category" . Though there is a category for the article. Even if I choose a category in frontend, after clicking save, the input box is empty and joomla returns the same message. If the assignments of the article are deleted, everything works fine again.
Could have something to do with the file:

components/com_content/models/form.php - which is model class for handling the form editation of article.

editing its function save() by adding there:

echo "something"; exit;

..will result in blank screen with the echo message, at least on articles without assignment. However on articles with association it completely omits this file - no reactions, even if i rename the file - which should result in error for missing file.
Never had this before Joomla 3.3.3! Backend works fine by the way! ALso, it has nothing to do with MENU association! The bug only affects ARTICLE association!

This bug was also reported by Tim Ziegeler here