Parent Package Configuration Error

Hi,
I have a child package running under main parent pakage.
I am passing value of integer & string type variables from parent package to child package's variables ( as disscussed in following artical http://msdn.microsoft.com/en-us/library/ms345179(SQL.100).aspx) ,values assigned in child package variables are futher used to build connection string for flat file connection manager in child package.

I have been using this whole SSIS package sucessfully for last three months,but suddenly the following error arises

"OnError,,,,,,,The result of the expression



Code Snippet
"@[User::StrTextFileImpDirectory] +"SomeTextStringHere"+ @[User::StrANTTextFileName] +(DT_STR,30,1252) @[User::Staging_Date_Key]+ "SomeTextStringHere"

on property "ConnectionString" cannot be written to the property. The expression was evaluated, but cannot be set on the property."




When I see the result of above mentioned expression through Log file than it appears that my string variable



Code Snippet"@[User::StrTextFileImpDirectory]



in child package has not been populated with any value from parent package variable, I.e it has been populated with empty string(While this is not desired value).

Same problem arises with my integer type variable



Code Snippet@[User::Staging_Date_Key]


,it has ben populated with value zero (i.e default value at design time for integer type),while this is not desired value.


Can some one guide me how to figure out the cause for error....
Is it possible that such kind of error may arise due to any change in enviornment of server....