Why does response code 500 started to appear in DataService request?

When I send the following JSON via DataService it fails:

{"Filters":{"rootSchemaName":"AccountCommunication","logicalOperation":0,"filterType":6,"items":{"filter1":{"LeftExpression":{"ColumnPath":"CommunicationType","ExpressionType":"SchemaColumn"},"ComparisonType":3,"FilterType":4,"RightExpressions":[{"Parameter":{"DataValueType":"Lookup","Value":"2b387201-67cc-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"},{"Parameter":{"DataValueType":"Lookup","Value":"6a3fb10c-67cc-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"},{"Parameter":{"DataValueType":"Lookup","Value":"9a7ab41b-67cc-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"},{"Parameter":{"DataValueType":"Lookup","Value":"ee1c85c3-cfcb-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"}]}}},"columns":{"items":{"Account":{"expression":{"columnPath":"Account","expressionType":"0"}},"Number":{"expression":{"columnPath":"Number","expressionType":"0"}},"Id":{"expression":{"columnPath":"Id","expressionType":"0"}}}},"RootSchemaName":"AccountCommunication","OperationType":"0","AllColumns":"false"}

Server returned HTTP response code: 500 for URL: https://esp.bpmonline.com/0/dataservice/json/SyncReply/SelectQuery

This JSON should simply read all AccountCommunication options of types Email, Fax, Phone, AltPhone.

When I remove even just one of the filter expressions (no matter which: Email, Fax, Phone or AltPhone) from the JSON, it runs fine:

{"Filters":{"rootSchemaName":"AccountCommunication","logicalOperation":0,"filterType":6,"items":{"filter1":{"LeftExpression":{"ColumnPath":"CommunicationType","ExpressionType":"SchemaColumn"},"ComparisonType":3,"FilterType":4,"RightExpressions":[{"Parameter":{"DataValueType":"Lookup","Value":"2b387201-67cc-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"},{"Parameter":{"DataValueType":"Lookup","Value":"6a3fb10c-67cc-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"},{"Parameter":{"DataValueType":"Lookup","Value":"ee1c85c3-cfcb-df11-9b2a-001d60e938c6"},"ExpressionType":"Parameter"}]}}},"columns":{"items":{"Account":{"expression":{"columnPath":"Account","expressionType":"0"}},"Number":{"expression":{"columnPath":"Number","expressionType":"0"}},"Id":{"expression":{"columnPath":"Id","expressionType":"0"}}}},"RootSchemaName":"AccountCommunication","OperationType":"0","AllColumns":"false"}

This doesn't make sense to me.

The error code started to appear about 2 weeks ago. Previously it worked fine.