Two users simultaneous authorization leads to Error 500 Apache Tomcat report

Preconditions
Log out if you were logged in.
There are user_1 and user_2.
There are 2 tabs opened: tab_1 and tab_2.

Steps to reproduce

1. Open 'Sign in' pop up dialog in both tabs
2. Enter user_1 data in tab_1
3. Enter user_2 data in tab_2
4. Click 'Sign in' button in tab_1
5. Go to tab_2 as soon as you can and click 'Sign in' button

Actual result
User_1 is not logged in, error 500 exeption report from Apache Tomcat is shown.
User_2 is logged in.

Expected result
User_1 is logged in. (*Or forum error 500 page is shown.)
User_2 is logged in.
(As it is when time between two authorization sessions is longer)

Video - http://screencast.com/t/9RgiSyOoP
Full error report - https://docs.google.com/document/d/1SrnBRCVTHKpmm5vrukYLOR3di00iCmbEwxHNiczKWqI/edit?usp=sharing
JCommune full session log:
2016-04-12 15:33:42 [DEBUG][-bio-8070-exec-5][anonymous-4D12][org.jtalks.jcommune.plugin.api.web.locale.JcLocaleResolver] - Parsed cookie value [en] into locale 'en'
2016-04-12 15:33:42 [DEBUG][-bio-8070-exec-5][anonymous-4D12][org.jtalks.jcommune.plugin.api.web.locale.JcLocaleResolver] - Added cookie with name [org.springframework.web.servlet.i18n.CookieLocaleResolver.LOCALE] and value [en]
2016-04-12 15:33:44 [DEBUG][-bio-8070-exec-4][user_1][org.jtalks.jcommune.plugin.api.web.locale.JcLocaleResolver] - Added cookie with name [org.springframework.web.servlet.i18n.CookieLocaleResolver.LOCALE] and value [en]