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Hmmm, probably caused by some characters in your image categories that the plugin doesn’t like. Any weird characters floating around? Apostrophes? Double quotes? Accents? Non-printable ASCII? Stuff like that?
thanks for your reply. i did check this before my post and didn’t find any. however, i’ll now re-enter all categories from scratch and see what happens. i’ll report back.
this is very weird: there were indeed strange characters in the categories (again   in the categories made by one of the editors), but they displayed only when i logged into the backend with opera (11.61). in firefox i couldn’t see it. therefore i didn’t find it first.
i did remove it and now i have the plugin back.
I think, there is a mistake on row where is
'step' => 'link_edit' . There should be
'step' => 'link' to browse links by categories on the same page.