Fatal Error: WriteCommentFormHeader()

after updating from 2.0.18 to 2.1.5 with traditional chocolate theme i get this error message.
somebody can help me with fixing it? i can open discussions, but i only see plain text on a white background and cant add comments.
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Please try switching to the default theme and see if the issue persists.
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hey, thanks!
I tried some things last night/morning and gave up. i switched to another theme and everything, except facebook plugin (the original vanilla one), works fine and i'm happy!
looking forward to fix the facebook bug too (i can log in via FB and share posts - but nothing apperas on FB. other users cant log in with FB)