Herewith some explanation regarding the BRAIN441 error which can occur when using formula variables in a query.
When a query uses a formula variable of the type “replacement path”, it can, for example, replace the occurrence of a characteristic with another value.

The BRAIN441 error will occur when the query doesn’t “know” how to replace the characteristic as it might occur in several InfoProviders, when using a query created on top of a Multiprovider.
As shown in the screengrab below, characteristic SSC_COKY occurs in all 3 Infoproviders (contained in the Multiprovider)

When you tick on all 3 checkboxes when identifying/assigning, the BRAIN441 error will occur when executing the query as it doesn’t “know” which characteristic to replace.
Should it replace SSC_COMPL__SSC_COKY (the green boxes) or should it replace SSC_COKY (the red box).
As long as both characteristics differ, the BRAIN441 error will keep on occurring.
The solution for the BRAIN441 error, in this case, is to only tick on the checkboxes which refer to the same characteristic. In this case, to tick on the characteristics in the green boxes.