I reinstalled Moon+ and I removed and readded the books. I've still the same error. Then I installed FBreader. It finds every tag and series perfectly. Though it has more problems than Moon+ at loading the included covers.
Yeah, I guess buying CC is an option. I probably will.
Anyway, in my book
Moon+ is a broken app even though it looks like one of the better reading apps. But tags being this broken is IMO inexcusable for a paid app.