OIC GEN3 external REST API error

When sending a GET request here,

https://payments.cat.uk.pt-x.com/payments-service/api/security/handshake

it works once or twice, then returns a 500 error every time after that. It worked fine in GEN2, and works fine in cURL and Postman (and even in a web browser).

I've tried setting the Connection: Close and Cache-Control: no-cache headers but still doesn't work. Error is as follows:

CloudInvocationException [ Fault Details :
<ns0:APIInvocationError xmlns:ns0="http://xmlns.oracle.com/cloud/generic/rest/fault/REST/Handshake">ns0:errorCode/http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.1</ns0:type>Internal Server Error</ns0:title>