I am not with Laminar Research; just a flight simmer.
There are many postings in this forum where Mac users are having trouble with High Sierra. Mac released this system update after the release of XP11 and it was determined the problem was with Apple and not with XP software.
As I am a windows user I don;t know if Apple have fixed the problem. I would suggest you go for the latest version of High sierra.
In addition I would suggest downloading the latest version of XP11. Although my XP system is mothballed I understand that XP11.20 has been officially released and a 11.30 beta version has been or is about to be released. I also understand that Laminar Research may have applied a partial fix for the High Sierra problem within version 11.20
If you are running steam (which is not the official Laminar Research version of X-Plane) you may have to wait some time to get the latest Xplane version.
By the way, attach your log.txt file to your question for others to interrogate to ascertain if there are other causes. Attach the complete file not a trunkcated version.