Answer:
This is a bug that occurred in version 5.10. It has been fixed in the current versions. The error was caused by a Windows update that set automatic deletion of temporary files and folders. The program has created a folder to store the temporary files or calculated results. Unfortunately, Windows deleted the folder before finishing the calculation. The program had nowhere to store the data and displayed the message: "Write Data() failed of module RF‑STEEL EC3." Unfortunately, the data cannot be restored. To resolve this issue, deactivate the Windows function deleting the unused temporary files or install a newer version of RFEM.
