Did you make any changes to the standard Zen Cart Configuration->Sessions settings?
Code:
Title 	                                        Value
Session Directory 	               {Your DIR_FS_SQL_CACHE directory} 	 
Cookie Domain 	                        True
Force Cookie Use 	                False 
Check SSL Session ID 	                False
Check User Agent 	                False
Check IP Address 	                False
Prevent Spider Sessions 	        True
Recreate Session 	                True
IP to Host Conversion Status 	true
Use root path for cookie path 	False
Add period prefix to cookie domain 	True