Error While Running Report !!

[gemini]

Hi,
On running a Cognos report using Cognos connection, we are getting this error:
Type a user ID and password
An attempt to connect to the data source failed.
* Indicates a required field.
-> Points to missing information.

Data source:
<Data source name>

User ID:
Password:
This report has been built using Cognos 8.4
This error is not permanent and after some time(hour or so) we don't get this error.
We don't have security applied yet.
Can anyone please let me know what could be cause of this error and how can we get rid of this.
Please do let me know if you need any more information.