Thanks for all the responses. After wireshark tracing, turns out to be a client side issue. Garbage packets/packets in wrong order are being sent by Chrome in rare occasions. Switching to HTTP/2 which seems to not have this issue in Chrome.by sverzijl - Other discussion
We're using Nginx in a reverse proxy configuration (with TIBCO AMX-BPM as a backend). In 99% of the GTW-RPC calls this works fine, but once every 100 or so requests (repeating same activity in browser over and over) we see that the request payload that was sent by the browser is being 'moved around/wrapped around' in Nginx when looking in the client_body_temp files. (The same issue is occurrinby sverzijl - Other discussion