In multithreaded call to a REST endpoint, while sending response, the code hits HeaderValueException intermittently

Issue 1:

While multiple calls are made simultaneously to an API, we hit this issue intermittently and seems to be a jersey issue.
Error message: Org.glassfish.jersey.message.internal.HeaderValueException: Too many "Content-Type" header values: "[application/json, application/json]"

This occurs at this line in code: Response.created(uriinfo.getAbsolutePathBuilder().scheme("https").path(newId.toString()).build()).build();

Issue 2:

While multiple calls are made simultaneously to the same API (let's say 30 calls are made to a REST endpoint) only 28/29 calls reach this API and it fails with NullPointerException. The code from our side seems fine.

Can not reproduce any of the 2 issues when a single call is made. Only reproduced over multiple threads are run together to make the calls simultaneously, that too intermittently.