Vaadin 7 and ProxyPass - Immediate Session Expiration

I'm having the same issue as https://vaadin.com/forum#!/thread/3620992. I am trying to get a Vaadin webapp to work through Apache ProxyPass. My setup is as follows:

• Web page (http://proxytest.com/) that proxies to the webapp
• The Vaadin webapp (7.5.2) is on https://testweb.com:5999/

I have added ProxyPassReverseCookiePath as one of the commenters suggested on the attached thread, but the I'm still getting a "session expired" alert immediately upon loading the page and am unable to go further in the webapp. It seems as though cookies are still not making it back to track my session so I'm wondering if I have improper syntax or some other issue. 

The browser console also shows that the webapp is falling back on long-polling after websockets fail.

My apache configuration is below.

Any help would be appreciated. Thanks!

<VirtualHost *:80>
ServerName proxytest.com
SSLProxyEngine On
ProxyPreserveHost on
ProxyRequests Off
</VirtualHost>

<LocationMatch "/">
ProxyPass  https://testweb.com:5999/
ProxyPassReverse  https://testweb.com:5999/
ProxyPassReverseCookiePath / /
</LocationMatch>