Pete, it probably depends on the XDA version you have. The site seems to be made with v18/19 so ought to be quite up-to-date.

This, however, might give you a clue: https://helpandsupport.xara.com/xara...d-devices.html
Open Sans has always been problematic on PCs, so this is just compounding your grief.

Recent XDAs now embed fonts directly into the website: https://helpandsupport.xara.com/xara...documents.html

Either update your XDA or search for the nearest Google Font to yours. The later XDAs now deliver Self-Hosted Google Fonts.

Acorn