"Could not find a valid session key" after subscription to event

After submitting a subscription form of an event (one subscription), users get the error:

"Sorry. A non-recoverable error has occurred.
(...)
Error type: Could not find a valid session key."

The suggestion in the error ('This error may indicate that users are accessing this page using a domain or URL other than the configured Base URL.') is implemented (the error exists since the upgrade to 3.4.5)

When the visitor want to subscribe multiple persons there is another error:
After submitting the form to subscribe the second users, we get a time-out and the first subscriber is added a lot to the civicrm contact list (> 900 times).

How can I debug this, is there a solution, ... ?