2 problems in your example code.

1) You should not register save handler as this code. Create handler object, then register it by sesion_set_save_handler() outside of the object. 

2) You should return "string" data from read(). Do you return "string"? It seems this is the reason why you get error from read()

For 1), you may register save handler methods like this, but intended usage is to register "object", not "methods".

Refer this test script for correct usage.
http://lxr.php.net/xref/PHP-7.0/ext/session/tests/session_set_save_handler_class_001.phpt

If you still have problem with modified save handler object, reopen this bug and paste whole save handler code.

Thanks for the input.  I am returning string data from read.  I've tried rewriting the handler with the code below based on the PHP documentation (http://php.net/manual/en/function.session-set-save-handler.php), but still getting the same warning (PHP Warning:  session_start(): Failed to read session data: user (path: C:\PHP\session\)).  Again, everything is working just fine, it is not failing to read the session data.  It can both read and write the session data fine from the DB, but it is returning the error that it cannot read from the file path since update to 7.1, even though it is correctly reading from the database.  Same code has worked fine on PHP 7.0 and every version in the last 6 years.

<?php
class DBSessionHandler implements SessionHandlerInterface
	{  
	public function open($save_path, $session_name) 
		{
     	return true;
   		}   
	
	public function close() 
		{
      	return true;
   		}    
		
	public function read($id) 
		{
		global $cid;
		// Set empty result
		$data = '';      
		// Fetch session data from the selected database  
		$sql = $cid->prepare("SELECT session_data FROM sessions WHERE session_id = ?");
		$sql->bind_param("s", $id);
		$sql->execute() or trigger_error("Session Error: " . mysqli_error($cid). " -- Query: " . $sql);
		$rs = $sql->get_result();	

		$a = mysqli_num_rows($rs);      
		if($a > 0) 
			{
			$row = mysqli_fetch_assoc($rs);
			$data = $row['session_data'];
			}      
		#trigger_error("read - " . $_SERVER['REQUEST_URI']);
		settype($data, "string");
		return $data;
   		}   
		
	public function write($id, $data) 
		{ 
		$time = time() + get_cfg_var("session.gc_maxlifetime");  

		$sql = $cid->prepare("REPLACE INTO sessions (session_id,session_data,expires) VALUES (?, ?, ?)");
		$sql->bind_param("ssi", $id, $data, $time);
		$sql->execute() or trigger_error("Session Error: " . mysqli_error($cid). " -- Query: " . $sql);

		#trigger_error("write - " . $_SERVER['REQUEST_URI']. " - " . mysqli_thread_id($cid));

      	return TRUE;
   		} 
		
	public function destroy($id) 
		{ 
		global $cid;
		
		$sql = $cid->prepare("DELETE FROM sessions WHERE session_id = ?");
		$sql->bind_param("s", $id);
		$sql->execute() or trigger_error("Session Error: " . mysqli_error($cid). " -- Query: " . $sql);

      	return TRUE;
   		}   
		
	public function gc($maxlifetime) 
		{
		// Garbage Collection
		global $cid;
		
	    $sql = 'DELETE FROM sessions WHERE expires < UNIX_TIMESTAMP();';      
		mysqli_query($cid, $sql) or trigger_error("Session Error: " . mysqli_error($cid). " -- Query: " . $sql);      
		// Always return TRUE

	    return true;
    	}
	}

$handler = new DBSessionHandler();
session_set_save_handler($handler, true);

session_start();
?>

Do you get read() error always or on occasion?
Could you check content of $data?

	public function read($id) 
		{
		global $cid;
		// Set empty result
		$data = '';      
		// Fetch session data from the selected database  
		$sql = $cid->prepare("SELECT session_data FROM sessions WHERE session_id = ?");
		$sql->bind_param("s", $id);
		$sql->execute() or trigger_error("Session Error: " . mysqli_error($cid). " -- Query: " . $sql);
		$rs = $sql->get_result();	

		$a = mysqli_num_rows($rs);      
		if($a > 0) 
			{
			$row = mysqli_fetch_assoc($rs);
			$data = $row['session_data'];
			}      
		#trigger_error("read - " . $_SERVER['REQUEST_URI']);
		if (!is_string($data)) var_dump($data); // <-- Something like this
		return (string)$data;
   		}

Looks like you were spot on.  Occasionally, the result from the DB would not return any data.  I configured the read function to return a blank string instead of NULL on empty results and it cleared up the error.  Strange this warning never fired on previous versions.

Thanks so much for your help.  Sorry to waste your time.