OpenSearch _reindex giving null_pointer_exception

I need to move an index from an ElasticSearch 6.8 domain to an OpenSearch 2.5 domain. They are both in a VPC.

For the ElasticSearch 6.8 domain in my account, I can see that I have an inbound connection under the Connections tab. For the OpenSearch 2.5, I can also see that I have the outbound connection with the aos-######.amazonaws.com link.

I run the reindex query:

POST /_reindex
{
  "source": {
    "remote": {
      "host": "https://aos-#########.us-west-2.es.amazonaws.com:443"
    },
    "index": "indexName-2023"
  },
  "dest": {
    "index": "indexName-2023"
  }
}

But receive back:

{
  "error": {
    "root_cause": [
      {
        "type": "null_pointer_exception",
        "reason": null
      }
    ],
    "type": "null_pointer_exception",
    "reason": null
  },
  "status": 500
}

I enabled the domain error logging and this is the stack trace:

java.lang.NullPointerException
	at __PATH__(StringBuilder.java:128)
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__
	at org.apache.http.protocol.ImmutableHttpProcessor.process(ImmutableHttpProcessor.java:133)
	at org.apache.http.impl.nio.client.MainClientExec.prepareRequest(MainClientExec.java:520)
	at org.apache.http.impl.nio.client.MainClientExec.prepare(MainClientExec.java:146)
	at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.start(DefaultClientExchangeHandlerImpl.java:129)
	at org.apache.http.impl.nio.client.CloseableHttpAsyncClientBase.execute(CloseableHttpAsyncClientBase.java:116)
	at org.apache.http.impl.nio.client.InternalHttpAsyncClient.execute(InternalHttpAsyncClient.java:138)
	at org.opensearch.client.RestClient.lambda$performRequestAsync$1(RestClient.java:421)
	at org.opensearch.client.Cancellable.runIfNotCancelled(Cancellable.java:93)
	at org.opensearch.client.RestClient.performRequestAsync(RestClient.java:419)
	at org.opensearch.client.RestClient.performRequestAsync(RestClient.java:406)
	at org.opensearch.index.reindex.remote.RemoteScrollableHitSource.execute(RemoteScrollableHitSource.java:190)
	at org.opensearch.index.reindex.remote.RemoteScrollableHitSource.lookupRemoteVersion(RemoteScrollableHitSource.java:112)
	at org.opensearch.index.reindex.remote.RemoteScrollableHitSource.doStart(RemoteScrollableHitSource.java:101)
	at org.opensearch.index.reindex.ScrollableHitSource.start(ScrollableHitSource.java:99)
	at org.opensearch.index.reindex.AbstractAsyncBulkByScrollAction.start(AbstractAsyncBulkByScrollAction.java:305)
	at org.opensearch.index.reindex.Reindexer.lambda$execute$0(Reindexer.java:156)
	at org.opensearch.index.reindex.BulkByScrollParallelizationHelper.executeSlicedAction(BulkByScrollParallelizationHelper.java:120)
	at org.opensearch.index.reindex.Reindexer.execute(Reindexer.java:136)
	at org.opensearch.index.reindex.TransportReindexAction$1.onResponse(TransportReindexAction.java:104)
	at org.opensearch.index.reindex.TransportReindexAction$1.onResponse(TransportReindexAction.java:101)
	at org.opensearch.index.reindex.BulkByScrollParallelizationHelper.initTaskState(BulkByScrollParallelizationHelper.java:158)
	at org.opensearch.index.reindex.Reindexer.initTask(Reindexer.java:131)
	at org.opensearch.index.reindex.TransportReindexAction.doExecute(TransportReindexAction.java:101)
	at org.opensearch.index.reindex.TransportReindexAction.doExecute(TransportReindexAction.java:58)
	at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:219)
	at org.opensearch.indexmanagement.rollup.actionfilter.FieldCapsFilter.apply(FieldCapsFilter.kt:118)
	at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:217)
	at org.opensearch.performanceanalyzer.action.PerformanceAnalyzerActionFilter.apply(PerformanceAnalyzerActionFilter.java:78)
	at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:217)
	at org.opensearch.action.support.TransportAction.execute(TransportAction.java:189)
	at org.opensearch.action.support.TransportAction.execute(TransportAction.java:108)
	at org.opensearch.client.node.NodeClient.executeLocally(NodeClient.java:110)
	at org.opensearch.index.reindex.AbstractBaseReindexRestHandler.lambda$doPrepareRequest$0(AbstractBaseReindexRestHandler.java:73)
	at org.opensearch.rest.BaseRestHandler.handleRequest(BaseRestHandler.java:125)
	at org.opensearch.rest.RestController.dispatchRequest(RestController.java:312)
	at org.opensearch.rest.RestController.tryAllHandlers(RestController.java:398)
	at org.opensearch.rest.RestController.dispatchRequest(RestController.java:241)
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__
__AMAZON_INTERNAL__

Any suggestions, or a way to get a more helpful error message would be very much appreciated. I am hoping to avoid using a proxy, since the point of the VPC connection thing is to no longer need a proxy. At the very least, I'd like to find a way to get a meaningful error message from the system.