Can you please paste the exact errors and check below things:
check following if its related to KMS:
1. IAM role policy and KMS policy should have allow permissions
2. Did you use extraConfig while mounting the source-s3 bucket:
If you have used IAM role while mounting the bucket. Verify iam-role-arn has KMS related permissions::
dbutils.fs.mount(
"s3a://src-bucket",
"/mnt/my-mount",
"sse-s3"
extraConfigs = Map(
"fs.s3a.credentialsType" -> "AssumeRole",
"fs.s3a.stsAssumeRole.arn" -> "iam-role-arn"
)
)