Calculated exch. rates between & and & are not in line with fixed rate &

What causes this issue?

An exchange rate type defined as a fixed rate is used for exchange rate
&V1& / &V2&. However, the amounts entered cannot be converted between
each other using this rate.
INCLUDE 'CUSTOMIZED_MESSAGE_INFO' OBJECT DOKU ID TX

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?

Adjust the amounts so that conversion with the fixed rate is possible.
The easiest way is to enter only one amount and initialize the other
amount field. The system then calculates the correct configuration
automatically.

Error message extract from SAP system. Copyright SAP SE.