Maggie, English is my native language and I also speak technobabble (I'm a computer geek, by profession) but I fully agree with you that the error message you saw is meaningless to a human being.
If you look at the 2 links in that message, you see that each one includes an ID number - that is for the profiles that are to be compared. It was one of those ID numbers that was apparently not found, for one of the reasons in the "note".
That same "note" also mentions "public database", which is a copy of the actual database. The links are to an external server, not to the WikiTree server and I don't understand why any link that would ever be used on WikiTree is being directed to a different server to get its data. At any rate, that is the reason for that error, which I think should never have happened. When you compare 2 profiles here, you should be comparing them on the real, live, database here, not on some copy of it that only gets updated weekly.
Eva's advice is very good - but, that is also a problem - when you choose a link to "compare", how are you supposed to know what server it is going to?????