Hi,
I don’t think so that this is authentication cookie issue, since if it would be, then you will be presented with logon page to log in. You can use the same Kentico account in two different browser instances (even different type of browser) without any issues.
I was not able to reproduce this behavior in version 6.0.13 so far. It might be really tricky to track this down, due to the fact that it is occurring only sometimes, am I right? Is there any related exception logged in event log (
CMS Site Manager -> Administration -> Event log)?
Do you have exactly the same problem as described by donal073-eircom or different one?
What is the “Maximum number of worker processes” setting in your case?
Thanks,
Miro