CI based web service error (Invalid UserID or Password)

I have created a component CI based web service, and call this web service by Java.

"Sometimes" this web service is not workable and return error message "IntegrationServiceClient: sendRequestToServer(). Invalid UserID or Password."

("Sometimes" means yesterday is OK but next day is not OK.)

I have checked the userid/password in "User Profiles" and they are "correct".

Does anybody know this issue and can point me in the right direction?

By the way, there is one options("Password Expired?") in this panel.

I tried to uncheck and check "Password Expired?" for this UserID and still get the same error message("IntegrationServiceClient: sendRequestToServer(). Invalid UserID or Password.")