Thank you for your detailed suggestions and guidance. After some investigation, I discovered that the issue was caused by the company's internal security program. I temporarily disabled the security measures, including antivirus software and controlled folder access, and the problem with saving .mat files was resolved.

I’ve confirmed that the "invalid argument" error no longer appears after performing the save operations you mentioned. Everything is now working as expected.

Thank you again for your support. Please let me know if you need any further clarification.

Best regards,
Junhyoek Jang