There are known issues with some of the builds from Microsoft Navision 4.0 SP2. Check the build number on the CFRONT.DLL and FIN.EXE. For most of the builds of C/Front, these must match.
You may want to update your C/Front to build 24080 (or later), which seems to be the most stable version of C/Front for SP2 & SP3 users and does not require a matching build of FIN.EXE. Using this build requires Jet Reports reports 5.3.1 or later.
You can find the latest version of Jet Excel Add-In in our download section
Refer to the article Which versions of NAV are compatible with Jet Reports .