After completing event: Catch Brand crystals and use them to disrupt the rift the Rift is opened and we can enter it.
But sometimes entering to opened rift - fails without any information.
After pressing F it starts loading instance, but after few seconds, loading screen changes back to original map image and send you back near opened rift.
Trying to enter to the same opened rift again - fails again.
If that happens you need to wait for a new rift.
So, is that some random bug, or maybe there is limit of max players to enter at the same time?