Issue Using Jet Reports with 4.0SP2
Solution
The version of C/Front released with 4.0SP2 by Microsoft will not work with Jet Reports.  If you intend on using Navision 4.0SP2 with Jet Reports please download and install the attached C/Front patch.

We have provided an updated batch of C/Front files that will enable the use of Jet Reports with your Navision 4.0 SP2 Client. These files are C/Front 4.0 SP1 files with the version information changed so that Jet Reports will accept them as being compatible with SP2. This is only intended to serve as a temporary solution until Microsoft resolves the issue and releases an updated version of C/Front.

http://zope.jetreports.com/Support/1150489986/index_html