Valuated stock becomes negative: & & & & -> see long text

What causes this issue?

When a goods movement or an incoming invoice is entered, the system
simulates the update of the valuation data in the material master
record. The current simulation indicates that this posting would lead to
a negative stock quantity (or a negative sales value) although
negative stocks are not allowed for this material.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

You cannot post this item. Therefore, terminate current processing.
Display the material master record (accounting data).
Check the price, stock value, and stock quantity (also for the previous
month, if required).
<DS:TRAN.MM03>Proceed</>
Display the material documents and the accounting documents for the
material and check whether the stock quantity is realistic.
Material documents <DS:TRAN.MB51>Proceed</>
Accounting documents <DS:TRAN.MR51>Proceed</>
If the stock value does not correspond to the stock quantity, data
inconsistencies may exist.
INCLUDE MB_NA_HOTLINE

Error message extract from SAP system. Copyright SAP SE.