elasticsearch fails VerifyRepositoryRequest in C# Nest client - elasticsearch

Question

After updating to elasticsearch 7.5.1 following code is now causing an exception
"Entry point not found"
//old code worked just fine
var verifyRepositoryResponse = this.elasticClient.VerifyRepository(new VerifyRepositoryRequest("C__MYFOLDER_Backup"));
//new code throws
var verifyRepositoryResponse = this.elasticClient.Snapshot.VerifyRepository(new VerifyRepositoryRequest("C__MYFOLDER_Backup"));
Note that making following request via fiddler
POST http://localhost:9200/_snapshot/C__MYFOLDER_Backup/_verify HTTP/1.1
User-Agent: Fiddler
Host: localhost:9200
Content-Type: application/json
Content-Length: 0
returns
{"nodes":{"1cWG9trDRi--6I-46lOlBw":{"name":"DESKTOP-5L01F6I"}}}
any ideas what causes the problem here?
Unfortunately there is zero documentation on that matter.
UPDATE:
Calling the code with a non existing directory as argument
var verifyRepositoryResponse = this.elasticClient.Snapshot.VerifyRepository(new VerifyRepositoryRequest("C__DOESNOTEXIST_Backup"));
I will get a 4xx response as expected in this scenario.
Before all tests, repositories and db have been cleaned.
Update:
Adavanced error information:
"# FailureReason: Unrecoverable/Unexpected BadResponse while attempting POST on http://localhost:9200/_snapshot/C__MYFOLDER_Backup/_verify\r\n - [1] BadResponse: Node: http://localhost:9200/ Exception: EntryPointNotFoundException Took: 00:00:00.1859931\r\n# Audit
exception in step 1 BadResponse:\r\nSystem.EntryPointNotFoundException: Entry point was not found.\r\n at Elasticsearch.Net.Utf8Json.IJsonFormatter`1.Deserialize(JsonReader& reader, IJsonFormatterResolver formatterResolver)\r\n at Deserialize(Object[] ,
JsonReader& , IJsonFormatterResolver )\r\n at Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)\r\n at Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Stream stream,
IJsonFormatterResolver resolver)\r\n at Elasticsearch.Net.DiagnosticsSerializerProxy.Deserialize[T](Stream stream)\r\n at Elasticsearch.Net.ResponseBuilder.SetBody[TResponse](ApiCallDetails details, RequestData requestData, Stream responseStream, String
mimeType)\r\n at Elasticsearch.Net.ResponseBuilder.ToResponse[TResponse](RequestData requestData, Exception ex, Nullable`1 statusCode, IEnumerable`1 warnings, Stream responseStream, String mimeType)\r\n at
Elasticsearch.Net.HttpWebRequestConnection.Request[TResponse](RequestData requestData)\r\n at Elasticsearch.Net.RequestPipeline.CallElasticsearch[TResponse](RequestData requestData)\r\n at Elasticsearch.Net.Transport`1.Request[TResponse](HttpMethod method,
String path, PostData data, IRequestParameters requestParameters)\r\n# Inner Exception: Entry point was not found.\r\nSystem.EntryPointNotFoundException: Entry point was not found.\r\n at Elasticsearch.Net.Utf8Json.IJsonFormatter`1.Deserialize(JsonReader&
reader, IJsonFormatterResolver formatterResolver)\r\n at Deserialize(Object[] , JsonReader& , IJsonFormatterResolver )\r\n at Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)\r\n at
Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Stream stream, IJsonFormatterResolver resolver)\r\n at Elasticsearch.Net.DiagnosticsSerializerProxy.Deserialize[T](Stream stream)\r\n at
Elasticsearch.Net.ResponseBuilder.SetBody[TResponse](ApiCallDetails details, RequestData requestData, Stream responseStream, String mimeType)\r\n at Elasticsearch.Net.ResponseBuilder.ToResponse[TResponse](RequestData requestData, Exception ex, Nullable`1
statusCode, IEnumerable`1 warnings, Stream responseStream, String mimeType)\r\n at Elasticsearch.Net.HttpWebRequestConnection.Request[TResponse](RequestData requestData)\r\n at Elasticsearch.Net.RequestPipeline.CallElasticsearch[TResponse](RequestData
requestData)\r\n at Elasticsearch.Net.Transport`1.Request[TResponse](HttpMethod method, String path, PostData data, IRequestParameters requestParameters)\r\n# Exception:\r\nElasticsearch.Net.UnexpectedElasticsearchClientException: Entry point was not found. --->
System.EntryPointNotFoundException: Entry point was not found.\r\n at Elasticsearch.Net.Utf8Json.IJsonFormatter`1.Deserialize(JsonReader& reader, IJsonFormatterResolver formatterResolver)\r\n at Deserialize(Object[] , JsonReader& , IJsonFormatterResolver
)\r\n at Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)\r\n at Elasticsearch.Net.Utf8Json.JsonSerializer.Deserialize[T](Stream stream, IJsonFormatterResolver resolver)\r\n at
Elasticsearch.Net.DiagnosticsSerializerProxy.Deserialize[T](Stream stream)\r\n at Elasticsearch.Net.ResponseBuilder.SetBody[TResponse](ApiCallDetails details, RequestData requestData, Stream responseStream, String mimeType)\r\n at
Elasticsearch.Net.ResponseBuilder.ToResponse[TResponse](RequestData requestData, Exception ex, Nullable`1 statusCode, IEnumerable`1 warnings, Stream responseStream, String mimeType)\r\n at Elasticsearch.Net.HttpWebRequestConnection.Request[TResponse](RequestData
requestData)\r\n at Elasticsearch.Net.RequestPipeline.CallElasticsearch[TResponse](RequestData requestData)\r\n at Elasticsearch.Net.Transport`1.Request[TResponse](HttpMethod method, String path, PostData data, IRequestParameters requestParameters)\r\n ---
End of inner exception stack trace ---\r\n at Elasticsearch.Net.Transport`1.Request[TResponse](HttpMethod method, String path, PostData data, IRequestParameters requestParameters)\r\n at
Nest.Specification.SnapshotApi.SnapshotNamespace.VerifyRepository(IVerifyRepositoryRequest request)\r\n at InforsHT.Genesis.Infrastructure.Data.Repositories.ElasticBackupAndRestoreProxy.IsBackupLocationPresent(String backupLocation) in
C:\\work\\eve\\Source\\InforsHT.Genesis.Infrastructure.Data\\Repositories\\ElasticBackupAndRestoreProxy.cs:line 44\r\n"

Answer 1

This is a bug. VerifyRepositoryResponse's Node property is attributed with a JsonFormatterAttribute that expects to deserialize an IDictionary<string, CompactNodeInfo> when the property type is IReadOnlyDictionary<string, CompactNodeInfo>. This is problematic for the underlying JSON serializer which uses strongly typed formatters, IJsonFormatter<T>, where the type T must match exactly.
I've opened an issue to track and a pull request to fix the issue. For now, you'd be able to workaround the bug by using the low level client exposed on the high level client
var response = client.LowLevel.Snapshot.VerifyRepository<DynamicResponse>("C__MYFOLDER_Backup");
and traverse DynamicResponse. Alternatively, use StringResponse and parse using JObject or JsonDocument and traverse.