Is your Synology running DSM 7.x? If so can you check Control Panel → Security → Advanced → TLS/SSL Profile level? What is it set to?
On DSM 6.x I used to have it set to the most strict, but I noticed on DSM 7.x that causes WebDAV to fail with Duplicati. I didn’t really dig too deep, but my guess is because it forced TLS 1.3 and mono has issues with it.
So for DSM 7.x you might need to set that to “Intermediate Compatibility”