Are requests being sent to your Wildfly server over HTTP or AJP? I remember having a lot of issues with the 100-continue header on Wildfly 8.2 and switching to HTTP stopped them. I never did find out the root cause of the issue. It may have been this, it may have been something else but it did go away when I upgraded to Wildfly 10. I used the switch in protocol as a temporary solution until I could upgrade. Hopefully this is of some help to you.
Thank you for the response. But I think your solution won't apply to us since the client request should already be HTTP.
The fix for that issue is this commit UNDERTOW-429 Make sure the continue response is not sent twice · undertow-io/undertow@8eceaf6 · GitHub
I guess, you could build undertow from source (choose the exact version that's used in your WildFly version) and apply this to it and use the new jar in your installation.
Thank you! I ended up doing something similar by applying the wildfly 8.2.1.Final patch from Downloads · WildFly, which contains the undertow update with the commit you mentioned. It went pretty smoothly so was a good solution to me.