If you experience this issue when using the PC*MILER|Spreadsheets add-in, it can be attributed to an anti-virus or anti-malware solution. If you whitelist the PC*MILER DLLs (C:\WINDOWS\pcmsrv32.dll & pcmsrv64.dll), that should resolve the issue.

NOTE: Whitelisting specifies the software applications that are authorized to run on your computer. Please check with your company's IT administrator if you are unsure of how to change your security settings to whitelist an application.