Things to check:
1) DefaultAppPool needs to be granted full control on the APP_Data folder
2) If you've upgraded to 18.104.22.168 from another version, make sure that the blog is set to 'multiusers' in web.config
<add key="BlogEngine.UsageScenario" value="multiusers" />
3) Check that the contents of users.xml, roles.xml and rights.xml are consistent (just open them with notepad to read them).
4) Errors can also be thrown if you've imported and are trying to update old blog entries that were written under a user name that no longer exists (I don't think that's happening in your case).
5) Make sure that the following is set correctly in your Web.Config file:
<modules runAllManagedModulesForAllRequests = "true">