Is it sinful to receive Communion at an SSPX Mass if it is not received with a schismatic mindset?
The priests of the SSPX do not have faculties to celebrate most of the sacraments, including the Eucharist. (Confession is an exception during the Jubilee Year of Mercy.) If someone chooses to attend and receive Communion at an SSPX Mass, knowing that the priests of the SSPX are being disobedient to the Church to celebrate that Mass when they do not have licit faculties, then that person chooses to support those priests in their act of disobedience. It may not be “schismatic,” technically speaking, but it can be participation in disobedience to the Church. Whether or not such support is “sinful” depends on the individual’s knowledge of the situation and choice to participate in spite of that knowledge.