Steps to replicate the issue (include links if applicable):
My QuickStatements tool uses stored OAuth permissions to edit Wikidata as the user. This has been working well for years, but since a few days (?) it does not work any more. I did not change any of the relevant code.
API error: "The authorization headers in your request are not valid: Invalid signature"
What should have happened instead?:
Correct edit (or any API request, really). Did something change on the WMF site?