Many chunks in world have had an invisible ceiling of darkness placed over them. Surface appears very dark, and mobs spawn during the day. Looking at the debug menu, you can see that the light level is 0 below a certain y-level, but immediately switches to 15 after you pass that height. The height is dependent on the highest block in that chunk. Making a 1x1 pillar to the build height limit caused the entire sky above the chunk to be at sky light level 0, but after the pillar was removed the y-level darkness threshold returned to its original level.

Problem persisted after switching machines, all clients experienced the effect on a server, and the issue did not change when the world was moved to a single player world. Do not know how to recreate issue, as it is the first and only occurrence so far.

Approximately 30 minutes prior to the issue we updated from Pre-Release 1 to Pre-Release 2 by switching the server.jar files. The update went smoothly, and no client had issues connecting. After a server host computer restart (for unrelated reasons), when the server was booted again is when the issue occurred.