Hi Colm,
You need first to understand which process trigger is that and where do you have it in place.
In theory the processtrigger with id... does not exist points to an orphan record, but it depends on its callstack, if you have something there to help you understand where had it been taken from it will be even better, otherwise the alternative is below.
The best recommendation I can provide for now is find a script online to search for a GUID in all tables, there are plenty available, the important thing is that should return all results and search for that GUID in the affected organization Database.
Once you have results you can decide what to do with it.
If you can not find any result and you are sure there should be at least one try to perform IIS reset, maybe the cache is the problem, I've seen this happening years ago and at the time asking my mentor he pointed to the cache on IIS level.
Keep in mind IIS reset has a small impact until the IIS is back online for your users, usually this impact won't exceed a couple of seconds.
Thanks!
