This morning I went to check your suggestion that the formula might be incorrect by having space or invisible characters in it. I got a shock when I opened my Clients database and found it empty, except for four records that cannot be seen. See 1st screenshot. I was able to retrieve a Time Machine backup and test that. It was strange in that the backups from yesterday were corrupted just like the one I originally opened. I had to go back a day or two to get one that was correct. It is also strange in that the corrupt file and the good file are at the exact same size, which seems to indicate that the records are there but just cannot be seen.
Once I got the Client database with all the records opened, I retested the formula. I used the Relational Workshop to recreate the formula and to ensure no extraneous characters were inserted, I used the copy button to past it into the formula field. I recalculated the field and got the same error as yesterday. See 2nd screenshot.
I did some further testing. I deleted the data in one of the Account Members fields. I then ran the Recalculate Field and the data was restored as expected. See 3rd screenshot.
At this point, I have no idea what is going on, although I suspect there is some corruption of the Clients database that may have caused the records to be hidden and the formula not to work. Looking forward to a resolution.
Thanks - Jim Weber