The file name property is not valid. The file name is a device or contains invalid characters.

I've started a very simple package to reade data from OLE DB source and save it as log files on a local path like C:\log.txt with different timestamps. I have created 3 variables to keep Filepath , Date and Fullpath as : 

Filepath=C:\ddd
@[User::date]=Replace(Replace(SubString((DT_STR,50,1252)getdate(),1,19),"-","")," ","_")
Fullpath=@[User::filepath]+"log"+"_"+ @[User::date]+".txt"

I dont have any complex environment (with proxy...)or any other special settings on my test environment and The Delay validation =true . Howevery, every time i run the package on SSIS ( not from SQL agent) i get the following error: 


Information: 0x4004300A at Data Flow Task, SSIS.Pipeline: Validation phase is beginning.
Error: 0xC0202070 at Package, Connection manager "Flat File Connection Manager": The file name property is not valid. The file name is a device or contains invalid characters.

Error: 0xC0202010 at Data Flow Task, Flat File Destination [2]: No destination flat file name was provided. Make sure the flat file connection manager is configured with a connection string. If the flat file connection manager is used by multiple components, ensure that the connection string contains enough file names.
Error: 0xC004701A at Data Flow Task, SSIS.Pipeline: Flat File Destination failed the pre-execute phase and returned error code 0xC0202010.
Information: 0x40043008 at Data Flow Task, SSIS.Pipeline: Post Execute phase is beginning.
Information: 0x4004300B at Data Flow Task, SSIS.Pipeline: "Flat File Destination" wrote 0 rows.
Information: 0x40043009 at Data Flow Task, SSIS.Pipeline: Cleanup phase is beginning.

Can you please let me know how i can solve this issue ? thanks