Common causes include:

• The client Spark version does not match the target Spark Connect App.
• The Spark Connect App corresponding to the serverId is currently unavailable.
• The target Spark Connect App has exited abnormally or is inaccessible.

Please check the following first:

• Whether the Java Client JAR matches the target Spark version.
• Whether the pyspark / py4j versions in the Python environment match the target Spark version.