Upload to Google Cloud Storage fails

I have a new instance of Nextcloud, with an external storage set up, pointing to a Google Cloud Storage bucket (using normal S3 intergation).
The files in the bucket are visible in Nextcloud, and I can delete them, but I can’t upload new files.
I have checked that this is not a permissions issue on the GCP side.
I’m very new to Nextcloud.

The error I’m seeing in the logs when trying to upload an empty text file named DUMMY.txt, not sure why it seems to be renamed DUMMY.txt.txt in the outgoing call to GCS.

Sabre\DAV\Exception: An exception occurred while initiating a multipart upload: Error executing "CreateMultipartUpload" on "https://storage.googleapis.com/tbyrne-cloud-s3/DUMMY.txt.txt?uploads"; AWS HTTP error: Client error: POST https://storage.googleapis.com/tbyrne-cloud-s3/DUMMY.txt.txt?uploadsresulted in a400 Bad Request response: <?xml version='1.0' encoding='UTF-8'?><Error><Code>InvalidArgument</Code><Message>Invalid argument.</Message><Details>PO (truncated...) InvalidArgument (client): Invalid argument. - <?xml version='1.0' encoding='UTF-8'?><Error><Code>InvalidArgument</Code><Message>Invalid argument.</Message><Details>POST object expects Content-Type multipart/form-data</Details></Error>