I had the same 400 error with IONOS S3 and it turned out to be a simple setup issue. In Cloudron’s backup settings, make sure you select the IONOS (Profitbricks) option instead of the generic S3 choice. Then double-check that the endpoint and region match your bucket exactly—for example s3.<your-region>.ionoscloud.com—and that path-style URLs are enabled. After I corrected those settings, the backups ran without any errors. If you still get the 400 after that, try a small manual backup or upload a tiny file with the same keys using the AWS CLI to confirm the credentials. If the problem persists, sharing the few log lines just before the crash will help others spot what’s wrong.
Hope this helps get your backups running smoothly.