Plone serving wrong content to user

I'm Samuele, a developer and supporter for a plone-based portal for the city of Padua,
We manage the support for this solution:

Plone version 4.1.6 with 2 server and 3 clients per server (each client with: zserver-threads 2) .

Zope Version
(2.13.15, python 2.6.8, linux3)
Python Version
2.6.8 (unknown, Feb 23 2015, 09:54:43) [GCC 4.7.2]
System Platform
Network Services
ZServer.HTTPServer.zhttp_server (Port: 8080)

Sometimes, in correspondency of high traffic a logged user, when come back from module filling service land in another user session. In the event log we see a lot of conflict error.

What could be the cause?

Other symptoms:

The url does not match the page content,
The user shown is the wrong one
Background image loaded incorrectly

Thank you

Strong doubts that Plone itself is in charge for delivering improper content. Even conflict errors should or better do not cause such a behavior. Are you doing caching using Varnish, Squid or whatever? I would search for the problem there. What kind of conflict errors do you see? Write or read conflict errors? Did you try to tackle the root of the conflicts down?

Apart from that: your setup is outdated with Python 2.6 being unmaintained for many years + an outdated Plone 4.1 installation. Upgrade to Python 2.7 + Plone 4.3.11.

The rest is wild guessing based on the limited information you provided. In doubt: get professional help from a someone with a deep understanding of Plone setups.