“An error occurred accessing a data source.”
Generally it is due to database or webservice connection. If you are getting this error
Then
1. First make sure the user has sufficient permission to access the database/web service.

2. In infopath ->Tools ->Form Options ->Security and trust ->Uncheck “Automatically determine security level” (If database/webservice is on same domain then select domain else select proper option)
3. Convert your data connection in udcx file.
For this, create a data connection library in MOSS.
Now open infopath form -> tools->Data connections
Select data connection and click on “Convert… “ button.
Give data connection library url with file name.
4. Now open Data Connection Library and Right click on udcx file -> “Approve/Reject” -> Select Approve and click OK.
5. Goto Central Administration ->Application Management->configure infopath form services->
“Allow user form templates to use authentication information contained in data connection files” option must be checked.

If your database/web service is placed on another domain then you need to check next option “Allow cross-domain data access for user form templates that use connection settings in a data connection file”.

Hope, It helps!!!