You will see this error when the encryption mechanism is set to SSE-S3/AES-256 but the configuration also declares an encryption key. The error happens because user-supplied keys are not supported in SSE-S3. Remove the fs.s3a.server-side-encryption.key setting or switch to SSE-KMS encryption.

java.io.IOException: AES256 is enabled but an encryption key was set in fs.s3a.server-side-encryption.key (key length = 44)
    at org.apache.hadoop.fs.s3a.S3AUtils.getEncryptionAlgorithm(S3AUtils.java:758)
    at org.apache.hadoop.fs.s3a.S3AFileSystem.initialize(S3AFileSystem.java:260)
    at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:3242)
    at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:467)