Conversion factors for units &1 and &2 faulty

What causes this issue?

Unit &V1& was successfully converted to unit &V2&.
The related conversion factors were also to be determined. An error
occurred in this, the cause of which is not known.

System Response

The system could not determine the conversion factors. As a result, the
conversion was cancelled.

How to fix this error?

Contact your system administrator who can analyze the cause of the
error.

Procedure for System Administrators

This is probably due to a program error.
The error is in function module 'MM_ARRANG_QUANTITY_CONVERSION'.
Function module &V3& was called. This call was not successfully.

Error message extract from SAP system. Copyright SAP SE.