Error during assignment of group & & to document number &

What causes this issue?

In the definition of the group for multiple processing, the reference
number must refer to the documents selected for the multiple processing
run (transfer requirements or delivery notes). The assignment is set
in a table (T311A). A system error occurred during the assignment in
this table.

How to fix this error?

STYLE S_DOCUS1
Contact your system administrator about the error. Note all paramters
in the error message as well as the concrete situation that caused the
error to occur.
STYLE *

Error message extract from SAP system. Copyright SAP SE.