Thanks Peter M. Yes, I tried that too, and agree. The problem is not with the DLL as such but with the HTB 2021 installation. Of course you have to keep changing the MSI Folder Path for the examples to run. DLLs RadioButton, CheckButton, Buttons and Cursor also fail. The ones involving message boxes tend to work. Because FileOpen is one of the first things encountered, I haven’t got far enough with HTB 2021 to find other run-time issues, but I have noticed that the FIND feature in the editor seems to be messed-up; highlighted text is not automatically placed in the search-box (which seems to be pre-filled with garbage).