Every time we try to xref in a 2018 stc file into our 2017 plant models the program locks up. This just started happening this afternoon. Has been working fine till now.
Have ran recover, etc.

I would recommend everyone being on the same platform/version. Going forward, i.e. a 2017 structure model in a 2018 plant model should be ok, but backwards can open up all kinds of gremlins.