This only happens in the production environment, not in acc-environment which is - or should be - exactly the same. Also it only happens with IE7 and IE8, not with modern browsers.
Already tried without success:
- suggestions in CMS7-6721 (flush hippo:log-entries)
- clear cookies
- checked tomcat versions and context.xml
- checked apache config
- IE compatibility mode
- IE: unchecked "Do not save encrypted pages to disk"
- checked catalina.out and hippo-cms.log while error occurs, no related messages logged