KNM v4 stores samples for each test into a proprietary database in the \statistics directory. The .nxr files contains the data, and the .nxi files are indexes created to speed up report generation. Sometimes the index files can be come corrupt and that can cause KNM not to find the data in the .nxr file when generating a report. When an index file has become corrupt its needed to manually delete it for KNM to re-generate it.
1) Shutdown KNM from the administrator console in web interface.
2) Delete the index files that has a date that indicates that its used in the report that you are generating from the statistics/ folder.
Caution! Do _not_ delete the files ending in .nxr. Those files contain the actual recorded data.
3) Restart KNM. This will cause KNM to re-create the missing index files.
4) Try to generate a new report. It should now be working as normal.