Hi,
We are not sure what is causing this issue as it is general .NET issue that might be related to your server settings. Maybe something was changed on your server regarding some settings.
If you use the Medium trust environment please follow these instructions:
http://devnet.kentico.com/docs/devguide/installation_in_medium_trust_environment.htm
http://devnet.kentico.com/docs/devguide/configuration_for_medium_trust_environment.htm
I would also recommend you to apply latest hotfix (5.5.27) (http://devnet.kentico.com/Bugtracker/Hotfixes.aspx) package over your 5.0 version of Kentico
CMS.
One of our customers was having the same issue. This how he was solving his issue:
"To resolve this, every time, we simply need to drag the web.config file OUT of the httpdocs folder (i.e. to the Windows desktop, for instance), then drag the web.config file back into the httpdocs folder. Then, after a long pause, the site will resume working normally again (until next time the problem occurs). It appears to be a permissions issue on the web.config file, that when the file is put back into the httpdocs folder, it's re-granted the inherited permissions from the httpdocs folder and the problem is resolved."
So, could you please also check the
disk permissions for your web application?
Best regards,
Juraj Ondrus