Thank you all for your responses. We took this issue to Mendix HQ in Rotterdam and there we came to the conclusion that the issue lies within the platform. When the loglevels are on INFO, it works. When the loglevels are set to TRACE/ DEBUG, it doesn't. This explains the inconsistent behaviour we've seen. Mendix confirms this issue within Mendix 7.7.1, I'm unaware of other platform releases where this issue might occur. Keep in mind that we're sending quite large XML messages and that small messages might not trigger this issue. Should you encounter something similar, check your loglevels and upgrade to the future version where Mendix has fixed the issue.
We experienced the same problem. We wanted to look into the request and response of a webservice and changed the loglevel of WebServices to trace. When we then looked into the log we saw the request and response xml. However BizTalk reported that we were sending requests with empty body’s. Our app is running on Mendix 6.10.10. This post helped us.
Are you sure this issue is solved in Mendix 7.1.1? I can’t find it in the release notes, but maybe I am a bad reader.