This error occurs when you are trying to reference a negative position in a string field.

Example:
  Gf.Gf_Solicitor_names = 'Blackbaud Inc.'

The functions below will return the following:

(Length({Gf.Gf_Solicitor_names}) = 14
Note: the length of the field is 14 characters.

instr({Gf.Gf_Solicitor_names},".") = 14
Note: the position of the period in the string is 14

Now if we use the following formula:
LastName := Right({Gf.Gf_Solicitor_names},(Length({Gf.Gf_Solicitor_names}) - instr({Gf.Gf_Solicitor_names},"."))-1)

This would be the same as saying the following:
LastName := Right({Gf.Gf_Solicitor_names},14 - 14 -1)
LastName := Right({Gf.Gf_Solicitor_names}, -1)

Trying to extract a negative string value from the {Gf.Gf_Solicitor_names} will generate this error. Adjust the formula so that negative string values are being referenced.