I think you still need to elaborate a bit on what you did try exactly, and if you are in the standard cloud or not (which make a huge diference).
First step would be to make everything work with soapUI.
Second step would be to make it locally work. Use Wireshark to debug the problems.
Third step would be to configurate the Mendix environment and add the certificates in the standard cloud. Step three is harder to debug. Most of the time you would need to do the logging / debugging on the customer side.
Hope this helps a bit.
Regards,
Ronald
Thanks for the post Ronald! I received some help from Mendix and now it works perfectly. He told me to redownload the certificate again by going to the base URL and downloading it manually. Before this I received the certificate by exporting it from SAP, which didn't work.