Amazon Athena MySQL connector: "Access denied for user '[USERNAME]'#'[IP]' (using password: YES)" - mysql

Question

I have a MySQL database instance in AWS RDS. I'd like to access it using AWS Athena. I used the "Amazon Athena Lambda MySQL Connector" to set up the new Data Source:
https://github.com/awslabs/aws-athena-query-federation/tree/master/athena-mysql
I installed this using the Serverless Application Repository. Here's the application:
https://serverlessrepo.aws.amazon.com/applications/us-east-1/292517598671/AthenaMySQLConnector
In the application settings before deploying, I used the same SecurityGroupIDs and SubnetIds as I used in another lambda function that is able to query the same database just fine.
In the environment variables for the lambda, I have the connection string set under both the key default and rds_mysql_connection_string (rds_mysql is the name of the Data Source in Athena). The connection string is in the format:
mysql://jdbc:mysql://HOSTNAME.us-east-1.rds.amazonaws.com:3306/DBNAME?user=USERNAME&password=PASSWORD
When I try to switch to the new data source in the Athena query editor, I get this error:
Access denied for user '[USERNAME]'#'[IP]' (using password: YES)
I diff'ed the role for the lambda that can connect against the one for the connector and they're pretty much the same. I even tried giving the connector the exact same role for a minute but it didn't help.
Using the test button on the lambda function for the connector also throws an error, but this could be a red herring. I use a blank test event and I get this in the logs:
WARNING: sun.reflect.Reflection.getCallerClass is not supported. This will impact performance.
Transforming org/apache/logging/log4j/core/lookup/JndiLookup (lambdainternal.CustomerClassLoader#1a6c5a9e)
START RequestId: 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a Version: $LATEST
2022-10-20 03:38:51 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a INFO BaseAllocator:58 - Debug mode disabled.
2022-10-20 03:38:51 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a INFO DefaultAllocationManagerOption:97 - allocation manager type not specified, using netty as the default type
2022-10-20 03:38:51 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a INFO CheckAllocator:73 - Using DefaultAllocationManager at memory/DefaultAllocationManagerFactory.class
2022-10-20 03:38:51 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a WARN CompositeHandler:107 - handleRequest: Completed with an exception.
java.lang.IllegalStateException: Expected field name token but got END_OBJECT
at com.amazonaws.athena.connector.lambda.serde.BaseDeserializer.assertFieldName(BaseDeserializer.java:221) ~[task/:?]
at com.amazonaws.athena.connector.lambda.serde.BaseDeserializer.getType(BaseDeserializer.java:295) ~[task/:?]
at com.amazonaws.athena.connector.lambda.serde.DelegatingDeserializer.doDeserialize(DelegatingDeserializer.java:56) ~[task/:?]
at com.amazonaws.athena.connector.lambda.serde.DelegatingDeserializer.deserializeWithType(DelegatingDeserializer.java:49) ~[task/:?]
at com.fasterxml.jackson.databind.deser.impl.TypeWrappedDeserializer.deserialize(TypeWrappedDeserializer.java:74) ~[task/:?]
at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322) ~[task/:?]
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674) ~[task/:?]
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3666) ~[task/:?]
at com.amazonaws.athena.connector.lambda.handlers.CompositeHandler.handleRequest(CompositeHandler.java:99) [task/:?]
at lambdainternal.EventHandlerLoader$2.call(EventHandlerLoader.java:899) [aws-lambda-java-runtime-0.2.0.jar:?]
at lambdainternal.AWSLambda.startRuntime(AWSLambda.java:268) [aws-lambda-java-runtime-0.2.0.jar:?]
at lambdainternal.AWSLambda.startRuntime(AWSLambda.java:206) [aws-lambda-java-runtime-0.2.0.jar:?]
at lambdainternal.AWSLambda.main(AWSLambda.java:200) [aws-lambda-java-runtime-0.2.0.jar:?]
Expected field name token but got END_OBJECT: java.lang.IllegalStateException
java.lang.IllegalStateException: Expected field name token but got END_OBJECT
at com.amazonaws.athena.connector.lambda.serde.BaseDeserializer.assertFieldName(BaseDeserializer.java:221)
at com.amazonaws.athena.connector.lambda.serde.BaseDeserializer.getType(BaseDeserializer.java:295)
at com.amazonaws.athena.connector.lambda.serde.DelegatingDeserializer.doDeserialize(DelegatingDeserializer.java:56)
at com.amazonaws.athena.connector.lambda.serde.DelegatingDeserializer.deserializeWithType(DelegatingDeserializer.java:49)
at com.fasterxml.jackson.databind.deser.impl.TypeWrappedDeserializer.deserialize(TypeWrappedDeserializer.java:74)
at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3666)
at com.amazonaws.athena.connector.lambda.handlers.CompositeHandler.handleRequest(CompositeHandler.java:99)
END RequestId: 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a
REPORT RequestId: 98cdd329-b0e5-49c4-8c9e-ae7b6f51dc2a Duration: 343.41 ms Billed Duration: 344 ms Memory Size: 3008 MB Max Memory Used: 170 MB Init Duration: 2665.96 ms