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.
I did find a page doing lessons just now - the weblio definition for research institute / 研究所 is consistently crashing the page whether it’s in lessons or on its own page
https://www.wanikani.com/vocabulary/研究所
https://www.weblio.jp/content/研究所
(just wanna say thanks for maintaining this script since i didn’t say so already, it’s really useful!! also sorry if there’s a better place to post than this thread lol)
Thank you for reporting this! (This is actually the perfect place for reports like these.)
The issue was that there was a script hidden in the part that I scraped from weblio and injected into the WK page, and WK did not particularly like that 
Anyways, I fixed the issue and uploaded version 0.9 to Greasyfork, you should be able to update it now.
Also I’m really happy to hear that the script is useful to you! Then I’m happy to maintain it. 
@Mods sorry for the misunderstanding, I should have made this clearer from the start:
I was actually talking about this thread’s OP 
Could you please turn it into a Wiki? I would like to add a link to the new version to it as I have taken over maintenance of the script from polv.
Oh oops, sorry. Ok updated!
Do you need my help, as I am not actively using WaniKani anymore?
I mean, can I unwatch?
I think we got everything sorted by now, thanks for checking back!
You mean, not watch this thread any more? Yes, I will take care of it moving forward.
Good luck with whatever your next steps in life are!
Thanks, feel free to @polv if you need any help.
Currently, I use less CDN; and tend to use build steps.
I just released v. 0.10 which additionally displays the kanji readings from Kanjipedia. They are inserted on the kanji, lesson and review pages.
Please shout out if you find any issues or dislike anything about it!