Every Monday we will post an entry that hasn’t yet been published with a view towards harnessing the collective onomastic power of the internet. If you have any thoughts about the name’s origin, other variants it might be related to, other examples of its use, etc., please share them in the comments! If you wish to browse other Mystery Monday names, there is an index.
Today’s name is so mysterious, we’re not even sure what gender it is. Grammatically, it looks to be feminine — it’s recorded in Latin and ends in -a in the nominative, which tips the evidence in favor of a feminine name — but the context provides no clear indication of the gender of the bearer, so we are still listing the gender as “unknown”.
It does appear to be a diminutive, with -ca or -ka being a moderately common feminine diminutive suffix in Slavic contexts, but what the root name might be, we haven’t a clue. Do you? Please share your thoughts in the comments!
Dictionary of Medieval Names from European Sources 
According to the index, this Mabca de Dobronicz is the sister of Stephanus de Dobronicz. The index shows another with this name, which also appears as Mabka and Mapka: Mapka, aunt of Hermannus de Ratišovice and husband of Janco de Ratišovice. The index to P. Ritter v. Chlumecky, Die Landtafel des Markgrafthumes Mähren, shows two more women named Mabca ~ Mabka from the later 14th and early 15th centuries. Zeitschrift des Vereines für die Geschichte Mährens und Schlesiens, vol. 2, p. 329, mentions a Mabka, the widow of Přibik von Bilowitz, 1399, and I found a few other examples. There seems to be no doubt that it is (or was) a Czech feminine name. The base normalized form should be Mabka; the p of Mapka pretty clearly results from assimilation to the voiceless k.
No source that I’ve found suggests any name of which it might be a diminutive, and in the documents in which the name occurs I’ve not found any feminine name of which it seems likely to be a pet form. Latin Amabilis ~ Amabilia did occur to me as a conceivable source, but so far I’ve no examples of it in that part of the world.
Browsing candidate names, there is not much left.
The hottest candidate is, IMO, Amabel. With some stretch, I can also imagine that Maura (via Mavra, as in Greek) can be turned into Mabka/Mabca as a diminutive.