Can you send your configuration for the sink minus any authentication credentials? Instead of writing just the integer/float value, wrap the value in a json document and set the value.converter in the a sink something like:
"value.converter": "org.apache.kafka.connect.json.JsonConverter",
"value.converter.schemas.enable": "false",
Alternatively, to get it working you could set the converter to string
"value.converter": org.apache.kafka.connect.storage.StringConverter"
By default the sink connector is looking for a BSON document you need to tell it to take whatever is on the kafka topic and interpret it as a string, a JSON document, etc…
Note: You can write the message with an Avro or JSON schema as well, then use
"value.converter": "org.apache.kafka.connect.json.JsonConverter",
"value.converter.schemas.enable": "true",