So I have placed a few fillers, put them in different modes. Have not connected to anything (no laser grid), just next to a block of bricks. I can continue to work in the world and log out, but I can't log back into the world - everything crashes.

This is a detailed (unlisted) video where I go through exactly what I have done. I used NBTExplorer to find and delete the problem so I could log back in, check video for all details of what I did and found. Unfortunately, I forgot to change quality in OBS back to 1080p... so it's downscaled to 720... it will be hard to read some of the text, but you have the crash report and you can see most of the text anyway. Oh, and sorry for being long and having a lot of background noise from the family as well ;)

Video:
http://youtu.be/7omh3Cn8yk0

Crash report:
http://pastebin.com/52d72QQK

Excellent, you found an actual, reproducible Filler bug! I'll get to it tomorrow.

Nice, hope it helps to provide video investigations. If not, tell me that crash report and a description is all required so I don't waste time on doing them ;)

And feel free to give techinal info, I'm always interested in how to interpret error messages, understanding how blocks works behind the scene and so on.