I have the same error either trying to install v.6.1.x from scratch or upgrading from a version prior to 6.1.
I asked my isp to check the permissions on folders and this was his response:

 The error indicates that it is trying to write to files on disk before ASP.Net impersonation kicks in, which can't be done as at that time it is still running as a user that doesn't have access to your domain files. File writing should never be done in any of the application startup functions, as these run before ASP.Net turns on impersonation (this is something Microsoft decided and we can't change). The best solution is for the developers to fix this bug so that it doesn't happen.

We can stop the error from occurring, but to do so we have to give permission for the application pool user to write to your domain. This means that any other website on the server can also write to it, which is obviously a security risk, so we don't recommend it and can not accept any responsibility. If you want to go ahead anyway, please let me know and I will make the change.

What should I do?

thanks

emanuele valenti