Stock value is negative: & -> see long text

What causes this issue?

The system has detected data inconsistencies in the material master
record:
The stock value is negative (&V1&), although negative stock balances
are not allowed for this material.

System Response

The system does not allow further movements for this material until
these inconsistencies have been corrected. Any further posting could
lead to even greater inconsistencies.

How to fix this error?

INCLUDE MB_NA_HOTLINE

Error message extract from SAP system. Copyright SAP SE.