Two problems that I can see:
1) The mapping may be fine - the error that you are seeing appears to be coming from the SCIM server that AAD Provisioning is talking to. AAD Provisioning sends a SCIM API request and the SCIM server is returning the InternalServerError. This needs to be investigated and debugged by whoever developed the connected application.
2) The attribute you have mapped to is not valid - the SCIM core user schema is defined in https://datatracker.ietf.org/doc/html/rfc7643 and does not include a "domain" attribute. You/the app developer will need to rename that attribute to use a namespace that isn't reserved by the SCIM specification - something like urn:ietf:params:scim:schemas:extension:appName:2.0:User:domain instead.