Thanks all for reporting this issue to our team right away. The fix is being deployed as I type, and should be live within the next hour!
Technical details for the curious
I broke this feature yesterday in release v2017-12-12.0 with this change: Web Lab: Environment-sensitive share URLs. I failed to take sharing from script levels into account in that change.
When the problem was reported today, we reverted the change, which is the fix going out in release v2017-12-13.0 right now.
I've also proposed this new change that reintroduces the Web Lab sharing fix, but does not break sharing from script levels and introduces lots of new test coverage for both sharing modes, so it won't happen again.