Elasticsearch 6.2 cluster snapshot process stuck with missing file

I have an AWS Elasticsearch 6.2 cluster whose snapshot process has been failing for several months, preventing new automatic or manual snapshots from being taken. The error in the logs seems to indicate that one of the snapshot files is missing from S3:

[2022-06-01T09:17:06,613][WARN ][r.suppressed             ] path: __PATH__ params: {pretty=, s=id, v=, repository=cs-automated}
SnapshotException[[__PATH__] Snapshot could not be read]; nested: SnapshotMissingException[[__PATH__] is missing]; nested: NoSuchFileException[Blob object [snap-9jgcTbzlSGKVwY6iFhDh5Q.dat] not found: The specified key does not exist. (Service: Amazon S3; Status Code: 404; Error Code: NoSuchKey; 
[...]

I had to add storage to the cluster, which replaced the existing cluster nodes, but this made no difference. It's not clear how to unblock this. Any ideas?