Looks like there is a lock on the number range set by one of the two programs (probably the background one). The other program tries to access the number range to get a new sales order number but fails.
You could solve this by changing your program where you first check if another instance is running already. Implement the singleton pattern.(ABAP OO) or call the lock mechanism for a report at the start of your program
See e.g. http://scn.sap.com/thread/1838118
If you want to try until the lock is finished, place the call to the enqueue function module in a while loop (on sy-subrc).