VisualEditor can't save edits (HTTP 404 error) when it was open for more than 24 hours while editing an old revision or after switching from wikitext.
Similar error also occurs in this scenario when trying to review your changes, or when trying to switch to wikitext mode.
This was originally discussed here: T233127#5573527 but I'm making a new task because that one got huge.
If you can't save your edit because of this bug, try the following workaround:
- Open your sandbox in the visual editor in another browser tab
- Copy-paste the entire page contents from the editor in the article to your sandbox – you may save the changes in your sandbox at this point, although it's not necessary
- Go back to the article, close the visual editor (discarding your changes), then open the visual editor again (make sure it does not restore your auto-saved edit)
- Copy-paste back from your sandbox to the article, then save your changes
- This workaround will overwrite any other edits on the page that happened in the meantime (without showing an edit conflict). Review the changes before saving to ensure you're not accidentally undoing others' edits.
- This workaround may cause dirty diffs throughout the page. If you remember which parts you edited, copy-paste only those parts to minimize this.