Crystal Report CRAXDRT error - logon failed

ERROR MESSAGE BOX:
Crystal Report Viewer
CRAXDRT Error Occured on Server. -2147189176 : Logon failed.
Details: [Database Vendor Code: 18456]

* Crystal Reports being run via ASP code & IIS
* Very intermittent error, cant find pattern - appears sometimes & not others.
* With some testing, found after a while or after running several diff crystal reports it seems to come good again OR when restart IIS (eg: Start -> Services -> 'IIS Admin Service' - restart).
* hourly auto updates are being done by IT on the server, not sure what changes/updates took place just before the problem started,
but customer does recall Crystal Reports was upgraded, think from version 9 to 11, so can dump into Excel. However this may have been some time ago.

Any ideas please?

* tried remapping datasource in all reports
* noticed many temp tables in code, some were changed to table variables

Relevant Software on Server:
* CR 8.5 (not in Start Menu though)
* CR 9 Developer
* CR 11 Developer
* Win Server 2003 SP2

This seems to fix it:
Open each problematic Crystal Report *.rpt file
File > Summary Info > Template: leave blank (was 'LOCALDSN' without the quotes)