So removing the winpaste line for the mouse in jstarrc fixed it - but no idea why. But I never would use that anyway, so this works for me. :)
I've been using joe in Unix and Linux for decades, and finally decided to try the Windows version. Win10, installed the .msi, and every time I type a capital M it tries to insert the clipboard, and gives me a 'Clipboard empty' on the bottom left if there's nothing there. Capital M is the only character that dosn't work correctly. Confirmed that all of the other characters work correctly. Not a clue where to start looking on this one...