We also had the same problem while accessing the web client. We opened a support ticket and the result was:
The user must have (read) access to his own user model!
So we changed the security string for the user itself (component details of the user) to something new like 'webaccess' and in access tab we give them a security community 'webaccess' with a read-only role. Now it is working.
When you use the default configuration (security string ADMIN for a user model) it will work if the user has also the security community ADMIN assigned with at least read access.