from @Ghent: "I just wanted to let you know I've been using irontanks 2.1.0 beta 2 and it seems to be going well. This shows up in my server log quite a bit, but seems harmless"

https://gist.github.com/Ghent/df3aa9e5e26335e1b93c758fc7aaf6f5

This appears to be caused by blocks that were placed prior to Beta 2 not containing all of the required nbt data. It is harmless as far as I can tell, but to get rid of the errors simply break and replace the blocks referenced in the error.