GitHub Webhook invalid X-Hub-Signature

I’m having a weird problem where the X-Hub-Signature that is sent in the header of the push request is invalid whenever the web app runs where it’s deployed, but it’s correct when the web app is running locally.
Could it be possible that there is some kind of platform dependency with the SHA1-calcuation or the like? It its very hard for me to pinpoint where exactly the problem lies.

Thanks in advance!