A newly created histogram will have its “number of entries” = 0. Each call to TH1::Fill and/or TH1::SetBinContent will automatically increase its “number of entries” by 1. This is NOT done when you call the TH1::AddBinContent method.
On the other hand, the TH1:Sumw2 will calculate new errors ONLY if the histogram’s “number of entries” > 0, so in your case, you need to call TH1::SetEntries in advance in order to “deceive” it (you may simply copy this number from your original histogram which you try to rebin).