A double angle cutter would be perfect for this https://www.harveytool.com/products/specialty-profiles/double-angle-shank-cutters But you asked about using the 1/4 ball endmill. If I open your op26 and select just the X+ hole instead of both for user defined edges, I get a good motion. For some reason it doesn't like the other hole. Maybe try recreating the geometry? It won't even do the X- hole if you select it by itself, kinda strange...

Sort of. It has been working all day today. The last thing I did to fix it was this; Close Mastercam then press the “Windows” button (at bottom left corner of screen), then type “reg” the search should show a result with “Registry Editor”. Open this item and navigate to “Computer\HKEY_CURRENT_USER\SOFTWARE\CNC Software, Inc.” Expand this folder and highlight the “Mastercam 2023” folder, right click it and select “Rename”. Change the name to “Mastercam 2023 old”. It came back after that, but it seems to be working all day today. We'll see if it sticks.