EAP + mod_proxy + SSL return a blank page to browser

Issue

We have configured Apache to use https for the connection to JBoss EAP.
Sometimes we see the expected results and sometimes not, i.e. parts of the page are missing or even nothing is shown at all.
If we configure Apache not to use ssl for the jboss connection these problems disappear.
Maybe it is useful to mention, that we call the apache only via https.