I'm happy to know that this problem is fixed for you, I just want to share my experience with this error while I developed the Word Add-in. Maybe the next person who get this error will find it helpful. I restarted my computer and the Add-in stopped working, I saw your error. My Word Add-in stack: Next.js, JS. IDE - Cursor. Mac. Clearing the Word cache helped effectively.

1. You need to close both Cursor and Word. Exit directly, quit, not just hide.
2. Then go to the Finder and find path (ctrl + cmnd + G) / Users / / Library / Containers / com.microsoft. Word / Data / Documents You need to delete the wef folder.
3. Then open Cursor and go to the main branch. npm i npm start
4. It worked.

I also tried:

• Office Add-in Validator Tool: Use the Office Add-in Validator to check the validity of your manifest file.
• Checked pathes inside manifest.xml and especially SourceLocation
• Created new certificates .crt, .key, office-addin-dev-certs
• Also tried deleting taskpane.html and have been doing npm run build.

Finally the way I described before helped to show my Word Add-in and work with it. But I need to mention that I cleared the cache several times. I had issues with branches. Word didn't wanted to show me my development progress and showed only version from the main branch. Then I cleaned wef and cleaned again, and in the end I got the result I needed.

Thank you for the attention!