SAP Message RSDD301 - Reading from aggregate &3 not possible due to block by user &1

Diagnosis
Data is read from aggregate &V3& in InfoCube &V2& for the creation ofan aggregate. To ensure that the data in aggregate &V3& has not changedduring the reading, an attempt was made to set a lock.
The aggregate is, however, already blocked by another transaction fromuser &V1&.
Either the data in aggregate &V3& is currently being created or userV1 is currently changing the definition of the aggregate.

System Response
Reading from aggregate &V3& is not currently possible.

Procedure
Check why aggregate &V3& is blocked by user &V1&, and start thetransaction again at a later point in time.