Just as I thought, Weblio has changed something on their pages. The issue
is should be fixed now. Thanks for reporting!
You can download Ver. 0.8 here: https://greasyfork.org/en/scripts/376666-wanikani-external-definition
or e.g. upgrade it on your Tampermonkey dashboard.
If you find any pages where the definition still does not get inserted, please feel free to post the links here so I can look into it again.