Hi Grant,
the most common reason for this error is, that the public key of the ticket creating system is either expired or does not exist in the PAB of the receiver. Accodring to the trace, you are using the SAPSYS.PSE.
I guess you did already compare the entries in the PAB of SAPSYS.PSE in client 800 with the contents of the PSE you created?
Kind regards,
Patrick