SAP Message VSB240 - External transmission &1 blocked

Diagnosis
You wanted to determine an internal transmission number for transferredor externally generated transmission number &V1&. The transmissionnumber generated externally could not be blocked.
The following exception occurred when trying to block the externaltransmission number: &V2&
The exception has the following meaning: &V2&
1 - The external transmission number is blocked
2 - A system error has occurred
3 - A different type of error has occurred
The transmission is assigned to transmission number 0 as a correctinternal transmission number could not be determined.
Try processing again as soon as the block on the external transmissionno longer exists.