epubcheck expects <meta property="mw:PageProp/notoc" about="#mwt1"/> to have a content attribute:
$ ./bin/console a:c -t 'Autor:Baltasar Gracián' -l es https://es.wikisource.org/wiki/Autor:Baltasar_Gracián ----------------------------------------------------- [WARNING] Line 23 column 58 of OPS/c0_Autor_Baltasar_Graci_n.xhtml: Error while parsing file: element "meta" missing required attribute "content" 22: </div><span about="#mwt1"> 23: </span><meta property="mw:PageProp/notoc" about="#mwt1"/><span about="#mwt1"> 24: </span><meta property="mw:PageProp/categorydefaultsort" content="Gracian" about="#mwt1"/>
As this tag is not used in the epub (and neither actually is the categorydefaultsort one below it in this example), it should be fine to just remove this element all together.