Nothing jumping out to me 
Server1 OS: Linux 2.6.32-358.18.1.el6.x86_64
Server2 OS: Linux 3.2.0-2-amd64
Both have the same settings for PHP sessions (other then default session.save_path)
Code:
Session Support enabled
Registered save handlers files user mm
Registered serializer handlers php php_binary wddx
Directive Local Value Master Value
session.auto_start Off Off
session.cache_expire 180 180
session.cache_limiter nocache nocache
session.cookie_domain .mystore.com no value
session.cookie_httponly On Off
session.cookie_lifetime 0 0
session.cookie_path /myadmin /
session.cookie_secure Off Off
session.entropy_file /dev/urandom /dev/urandom
session.entropy_length 32 32
session.gc_divisor 2 1000
session.gc_maxlifetime 900 1440
session.gc_probability 1 0
session.hash_bits_per_character 5 5
session.hash_function 0 0
session.name zenAdminID PHPSESSID
session.referer_check no value no value
session.save_handler user files
session.save_path /home/user/cache /tmp
session.serialize_handler php php
session.upload_progress.cleanup On On
session.upload_progress.enabled On On
session.upload_progress.freq 1% 1%
session.upload_progress.min_freq 1 1
session.upload_progress.name PHP_SESSION_UPLOAD_PROGRESS PHP_SESSION_UPLOAD_PROGRESS
session.upload_progress.prefix upload_progress_ upload_progress_
session.use_cookies On On
session.use_only_cookies On On
session.use_trans_sid 0 0
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