the last version of the Sen2Cor plugin (6.0.1) is not yet compatible with the latest released version of Sen2Cor (v2.5.5). We are working on it, but at the moment, the only way for using Sen2Cor 2.5.5 is via the command line. As soon as we have fixed this problem we will release a module update solving it.
This incompatibility affects only to the execution of Sen2Cor via SNAP, not to opening of the L2A outputs by SNAP. The L2A products generated by every version of Sen2Cor (including 2.5.5) are supported by the last released version of SNAP.