Starting with v. 2.1 of Esprit, the RTIfcClient.dll file has been renamed to Bruker.API.Esprit.dll
PFE still looks for the original RTIfcClient.dll file name, so you will need to rename the new file.
After renaming, the file should be placed in the SysWOW64 folder so that all PFE/PI apps can access it.
It is perhaps sensible to remove all occurrences of the older version of RTIfcClient.dll before renaming the new version.
Gareth