I can view a profile, view all the activities, but when clicking 'view' to view the details for the individual activity I get 'Network error, unable to reach the server. Please refresh this page in your browser and try again'. Any thoughts? I've restarted the machine, logged in/out etc, I have privileges to access the information, made sure it's a secure connection, everything I can think of. Any help would be great!
Symptoms like you describe match the symptoms of a bug in the Activity iCalendar Feed extension, which has been fixed as of extension version 1.5.
If you've got this extension installed, I recommend a) disabling it temporarily to see if this bug could be at fault, and b) upgrading it to the latest version. (Hat tip to V3rmie for creating the ticket for that bug.)