The issue is no longer relevant; there are new symptoms that will be fixed in upcoming tickets:

This one can be closed.

Is it still blocked? Could you proceed with implementation?

CC

I have just tested the following endpoint on the Eureka BF environment, and the issue persists. I am receiving a 500 error

https://edge-eureka-bugfest-ramsons-consortium.int.aws.folio.org/patron/account/ssbuck@ebsco.com/instance/f4cc09f3-8340-581a-ac1b-a9284c0bedf4/hold?apikey=eyJzIjoiYm5PUWN4dVZabyIsInQiOiJjczAwMDAwaW50IiwidSI6IkVCU0NPRWRnZSJ9
Other endpoints are working fine, for example:
https://edge-eureka-bugfest-ramsons-consortium.int.aws.folio.org/patron/account/123/item/464d94ce-4d35-5e61-8cfd-efc21b011fe1/allowed-service-points?apikey=eyJzIjoiYm5PUWN4dVZabyIsInQiOiJjczAwMDAwaW50IiwidSI6IkVCU0NPRWRnZSJ9

issue will be retested after fix

Still getting a 500 from mod-patron.  It seems to receive 2xx responses from other modules and, even more, it creates an ECS request successfully - example . investigation in progress