I think I found the reason for my problem: since this is a family return my wife and myself both applied for Ontario Trillium Benefit (OTB) on the ON-BEN Aplication for tax credits page. For both the box was showing Yes. This is obviously wrong - the moment I unchecked the box in my wife's Interview (box empty), choose NO or simply deleted the ON-BEN application in my wife's Interview, the Energy/Property credit (line 61020) appeared in my return and Ontario Trillium Benefit (OTB) have been calculated properly.
I believe this should be corrected by the program automatically - the moment the head of the family (or one of the family members) applies for ON-BEN tax credits in the interview, the other family member must receive a warning or not have the chance to apply at all.