Hi @ShlomiD, welcome to the forum.
Duplicati up to version 2.0.8.1 was using a nonce-based authentication system, version 2.1.0.2 uses a JWT based authentication system.
I am not sure if Heimdall is updated to support the JWT authentication approach, but the error message in the screenshot looks like Heimdall is attempting to call the nonce API which is not present in 2.1.0.2.
If this is the case, it must be fixed in Heimdall, so you can log an issue there.