Quantity Conversion Interface: No response from external programs

What causes this issue?

The external c-codes (API or AGA or customer functions) do not respond
to the quantity conversion interface (QCI). No results have been
imported back into the SAP system from the external c-code routines.

System Response

This might be a "time-out response" on the part of the system, or it
could be that one or more of the function routines cannot be found by
the system, or that insufficient data has been passed to the external
function.
CHECK IF THE EXTERNAL ROUTINES HAVE BEEN INSTALLED PROPERLY: Run Report
ROIB_QCI_CALL_TEST to check the installation, read the documentation of
this report for more details

Error message extract from SAP system. Copyright SAP SE.