Quick add new term not working in Studio 2019: error message: attempted to divide by zero

I'm suddenly finding I'm unable to add new terms to a glossary as I work on Studio 2019. Every time I highlight both source and target terms I get an error message "attempted to divide by zero." I've tried closing and re-opening the program and rebooting the computer, but no joy. I've also reorganised the glossary in Multiterm, all to no avail. Does anyone have any suggestions? I can't think I've done anything different to make it stop working... This happens with both quick add and add new term.
Many thanks.