New bug in Origin 2020.

1. Mouse over legend so hand cursor shows.
2. Click so it enters border mode. (double border shows with handles)
3. Drag the bottom-right corner of the whole legend box (double border) and drag diagonally.

  ==> Origin crashes.

Workaround: Don't drag diagonally. Drag horizontally or vertically.

Or right click legend and choose Properties.

Go to Frame tab to adjust Margins.