Use envelopes that have been stored in a dry environment.
Note: Printing on envelopes with high moisture content can seal the flaps.
Resend the print job.
Does the envelope seal when printing?
Contact customer support.
The problem is solved.
Incorrect Staple Postion on the Lexmark C950 and X950, X952 and X954 Printers and MFPs
Lexmark C/XC41xx and CS/X72x: 200.13 Paper Jam Error and/or Grinding Noise when Printing