Error starting process instance by REST using Openshift

I have a war file (built using the servlet war Maven archetype) deployed to Camunda 7.12 on Openshift Container Platform. When I try to start a new process instance using the URL “https://{hostname}/engine-rest/process-definition/key/{processkey}/start” using Python’s Requests library, I get the following error:

java.lang.IllegalStateException: Cannot create a session after the response has been committed

My two best guesses for the source of this error are 1- the specified engine is wrong in my REST call is wrong, or 2- I need to convert my process into a new Maven project with a Spring boot archetype. Can anyone point me in the right direction? Thanks in advance!