The error may be caused by one of the following reasons:
1. Number assignment does not function correctly and the system attempts to assign the same number twice.
2. A report blocks VBAK completely. Thus you should find out whether the same report is always run parallelly if this error occurs or if the current job may cause such a block.
3. The block table is full so that the system cannot block another document. A large number of block entries (which can be listed with Transaction SM12) could point to this cause.