Description of problem:

After migration from 3.6 EL6 (with passing through 3.6 EL7) login page ends in error with code 500.

~~~	
Operation Canceled
Error while executing action: A Request to the Server failed with the following Status Code: 500
~~~

I see in server.log

~~~
2016-04-25 17:08:16,760 ERROR [io.undertow.servlet] (default task-32) Exception while dispatching incoming RPC call: com.google.gwt.user.client.rpc.SerializationException: Can't find the serialization policy file
. This probably means that the user has an old version of the application loaded in the browser. To solve the issue the user needs to close the browser and open it again, so that the application is reloaded.
        at org.ovirt.engine.ui.frontend.server.gwt.GenericApiGWTServiceImpl.doUnexpectedFailure(GenericApiGWTServiceImpl.java:176) [frontend.jar:]
        at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:67) [gwt-servlet.jar:]
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:707) [jboss-servlet-api_3.1_spec-1.0.0.Final.jar:1.0.0.Final]
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) [jboss-servlet-api_3.1_spec-1.0.0.Final.jar:1.0.0.Final]
        at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:85) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:129) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at org.ovirt.engine.core.utils.servlet.HeaderFilter.doFilter(HeaderFilter.java:94) [utils.jar:]
        at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at org.ovirt.engine.ui.frontend.server.gwt.GwtCachingFilter.doFilter(GwtCachingFilter.java:132) [frontend.jar:]
        at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at org.ovirt.engine.core.branding.BrandingFilter.doFilter(BrandingFilter.java:73) [branding.jar:]
        at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at org.ovirt.engine.core.utils.servlet.LocaleFilter.doFilter(LocaleFilter.java:66) [utils.jar:]
        at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
        at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:84) [undertow-servlet-1.3.15.Final.jar:1.3.15.Final]
~~~

Version-Release number of selected component (if applicable):
ovirt-engine-webadmin-portal-4.0.0-0.0.master.20160423161403.gite38df80.el7.centos.noarch

How reproducible:
100%

Steps to Reproduce:
1. el6 3.6 -> el7 3.6 -> el7 4.0 master
2. go to login page
3.

Actual results:
~~~	
Operation Canceled
Error while executing action: A Request to the Server failed with the following Status Code: 500
~~~

Expected results:
should work

Additional info: