No action necessary, just writing this down in case it helps anyone else.
Uploading via SimpleBatchUpload might fail with "ERROR: Invalid token", confirmable from watching network requests in browser devtools for a 200-code response to upload POST requests that notes the invalid token in the response body.
It might happen if user groups or rights for that user are modified during an active session. Logging out and logging back in does not resolve the problem; you must also clear all browser cookies for the wiki in question.