I try to add a screen with a name which already exists.
The trace :

20: url: http://zabbix.krg/zabbix/api_jsonrpc.php
10: Trying to login with 'api':'md5(8a5da52ed126447d359e70c05721a8aa)'
10: json_obj: {'params':

{'password': 'api', 'user': 'api'}

, 'jsonrpc': '2.0', 'method': 'user.authenticate', 'auth': '', 'id': 0}
10: Connection object <httplib.HTTPConnection instance at 0x260a5f0>
20: Sending: {"params":

{"password": "api", "user": "api"}

, "jsonrpc": "2.0", "method": "user.authenticate", "auth": "", "id": 0}
10: Sending headers:

{'Content-Type': 'application/json-rpc', 'User-Agent': 'python/zabbix_api'}

20: Response Code: 200 OK
10: Response Body:

{'jsonrpc': '2.0', 'result': 'b99fef7c417cad2fac5559c78f2459de', 'id': 0}

10: json_obj: {'params': {}, 'jsonrpc': '2.0', 'method': 'APIInfo.version', 'auth': 'b99fef7c417cad2fac5559c78f2459de', 'id': 1}
10: Connection object <httplib.HTTPConnection instance at 0x260a878>
20: Sending: {"params": {}, "jsonrpc": "2.0", "method": "APIInfo.version", "auth": "b99fef7c417cad2fac5559c78f2459de", "id": 1}
10: Sending headers:

{'Content-Type': 'application/json-rpc', 'User-Agent': 'python/zabbix_api'}

20: Response Code: 200 OK
10: Response Body:

{'jsonrpc': '2.0', 'result': '1.2', 'id': 1}

10: json_obj: {'params':

{'sessionid': 'b99fef7c417cad2fac5559c78f2459de'}

, 'jsonrpc': '2.0', 'method': 'user.checkAuthentication', 'auth': 'b99fef7c417cad2fac5559c78f2459de', 'id': 2}
10: Connection object <httplib.HTTPConnection instance at 0x260a998>
20: Sending: {"params":

{"sessionid": "b99fef7c417cad2fac5559c78f2459de"}

, "jsonrpc": "2.0", "method": "user.checkAuthentication", "auth": "b99fef7c417cad2fac5559c78f2459de", "id": 2}
10: Sending headers:

{'Content-Type': 'application/json-rpc', 'User-Agent': 'python/zabbix_api'}

20: Response Code: 200 OK
10: Response Body:

{'jsonrpc': '2.0', 'result': True, 'id': 2}

10: json_obj: {'params':

{'vsize': '2', 'name': ' Test screen', 'hsize': '3'}

, 'jsonrpc': '2.0', 'method': 'screen.create', 'auth': 'b99fef7c417cad2fac5559c78f2459de', 'id': 3}
10: Connection object <httplib.HTTPConnection instance at 0x260a9e0>
20: Sending: {"params":

{"vsize": "2", "name": " Test screen", "hsize": "3"}

, "jsonrpc": "2.0", "method": "screen.create", "auth": "b99fef7c417cad2fac5559c78f2459de", "id": 3}
10: Sending headers:

{'Content-Type': 'application/json-rpc', 'User-Agent': 'python/zabbix_api'}

20: Response Code: 200 OK
10: Response Body: {'jsonrpc': '2.0', 'id': 3, 'error': {'message': 'Application error.', 'code': -32500, 'data': 'Attempt to close not started transaction from: CScreen::create'}}
Error -32500: Application error., Attempt to close not started transaction from: CScreen::create

The message is not really explicit / may reflect some other issues.