Для вызовов асинхронных методов транзакции не поддерживаются. Задайте синхронный вариант метода или восстановите исходное значение свойства TransactionOption атрибута WebMethodAttribute.

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Transactions are not supported for asynchronous method calls. Provide a synchronous version of the method or unset the TransactionOption property of the WebMethodAttribute.

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