[TYPO3-mvc] sys_category in own extension not working after 6.2 upgrade

[Jan Kornblum]

Hi newsgroup,

i've developed an extension which makes use of sys_category for a 
certain domain object. The field used for categorization by 
"MakeCategorizable()" is called "categories". Everything worked fine in 
6.1.

After an upgrade to 6.2.x, the assigned categories are not longer shown 
in the backend editing forms category tree (all checkboxes are 
unchecked, the tree itself is rendered correctly). The relations 
themselfes still seem to exist, as in FE everything still matches.

Can anybody give me a hint about what might be wrong?

Kind regards, Jan