I was just entering a ticket for the same issue using Digital Ocean. I am also seeing the [InvalidAccessKeyId] error in failed backups for multiple websites. Example log excerpt below:
0054.156 (0) S3-Compatible (Generic): Error: Failed to access bucket [redacted]. UpdraftPlus_S3::putBucket([redacted], private, us-east-1): [InvalidAccessKeyId] The AWS Access Key Id you provided does not exist in our records. (line: 491, file: /srv/users/[redacted]-prod/apps/[redacted]-prod/public/wp-content/plugins/updraftplus/includes/S3.php)
0054.157 (0) An error condition has occurred for the first time during this job
...
0726.412 (3) There were errors in the uploads, so the 'resume' event is remaining scheduled
0726.413 (3) The backup has not finished; a resumption is scheduled
The backups ultimately fail after multiple retries.
FYI, I also see a MySQL error in the logs [Binary mysqldump: error (code: 5)] which doesn’t seem to kill the update workflow. I see other users are experiencing it too: https://updraftplus.com/forums/topic/binary-mysqldump-error-code-5/