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:
- View the same page in a new browser tab
- Open the editor in this tab (make sure it does not restore your auto-saved edit – if it does, close the editor, discard your changes, and open it again)
- Copy-paste the entire page contents from the editor in the the old tab to the new tab
- Save in the new tab
- 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.