Since ownCloud 10.4.1 we run into a bug that federated shares cannot be accepted, when the Music player is enabled.
It seems that the user session is broken, so that the user session element is no longer correctly passed.

How to reproduce

1. Create a new federated share from an another owncloud to an owncloud 10.4.1 with music player enabled.
2. Try to accept the share via the notification pop-up or via "Shared-with-you" option in the menu at the left side.