The issue with MCNPX was related to a missing cross-section file, specifically "bertin," which caused the error message "bad trouble in imcn." This typically indicates an input error, suggesting that the necessary file was not found. The user resolved the problem independently, although the specific steps taken to fix it were not detailed. Missing files are a common source of calculation errors in MCNPX. Ensuring all required files are present is crucial for successful operation.