Workaround for the "Creator not found" Error
You can now view those posts!
Cache Metadata of Posts from Affected Creators
Creator Posts page will show posts that you have previously viewed.
Silent Retry on HTTP 429
If you still see a 429 error page, it means all 3 retry attempts have failed.
Rename Any Creator
Access this from your userscript manager menu. You can rename any creator, not just the "not found" ones.
Show Full Imported Date with Time
Any Pixiv Fanbox post will still show "Unknown" as the raw data from api is null.
Note
This will NOT enable you to see all posts on Creator Posts page, only the ones you've visited.
Try the "< previous" and "next >" links at the top of Post Details page, they work ...to some extent.
Important Info
Make sure you have "Synchronous page mode" enabled in Violentmonkey advanced settings.
For other userscript managers, look for similiar option that allow the script to run as early as possible.
Avoid having multiple tabs/windows open on the "not found" creators, I'll try implementing a lock mechanism soon™