Steps to reproduce:
1) Create a new project.
2) Enable Collab.
3) In the Collab popup, press on 'Invite Teammate' (three people) icon.
- Alternatively, you can go to Window > Lighting > Light Explorer, but there's a lower chance to reproduce the issue this way.

Errors start appearing in the Console every 4-7 seconds:
Assertion failed: Assertion failed on expression: 'm_SendState.state == kSendingNotStarted'

The errors seem to do no harm, however, once they start appearing, only restarting the Editor helps disabling them.
The error seems sometimes occurs with other Editor windows as well.

Reproduced on:
2017.2.0a1
(Jun 6th, 2017) 2017.2.0a3 (72741632a6a6)

Did not reproduce:
2017.1.0b7, 5.6.1p1

Tested on:
Windows 10
macOS 10.12.5