Greasy Fork is available in English.
Highlight or hide works you kudosed/marked as seen.
< Отзив за AO3: Kudosed and seen history
The local storage in your browser has limited capacity, but normally it should remember around 200,000 works you mark as seen. If you're using other userscripts for AO3, they may be filling up that space so there's less left for your seen list. Other than that it's not enough info to troubleshoot, sorry.
much appreciated, thank you!
Seen List Full?
Love this script, but it's stopped saving works marked as seen unless I unmark other works