Hi, I'm using the K2 User module to manage user login and registration on my site artedelviaggio.com . Everything works fine, except that when a logged-in user clicks on the "My account" link, they get the following error:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, This email address is being protected from spambots. You need JavaScript enabled to view it. and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

This may be a permissions error.
backup your site (akeebabackup)
then use akeeba admin tools to reset all the permissions as to default as they should be