I'm experiencing the same issues as Nigel Todman. Some further info:
Happens when I join a server (notably a private survival), or after a few minutes of play.
At one point the debug window was spammed with over 5000 of those 'error executing task' 'NullPointerException' messages, with varying class names (jh, jc, etc.). Only happened once.
It isn't consistent. Sometimes it kicks me instantly, sometimes I can move for a few seconds, sometimes I can play for extensive periods of time. It also seems to depend on location. It occurs mostly in my base. I've never seen it happen in the nether. Don't know if this is actually the case, just some notable observations.
Occurs in bouts. Sometimes there's an episode of a couple dozen times, sometimes it doesn't happen.
It appears to not be a server side issue as there are no messages on the server, only on the client, and only that one time.
Things I've tried:
Renaming .minecraft for a reinstall of the launcher info.
Logging out, then back in, in the launcher.
Logging out, then back in, on my computer account. Seemed to temporarily pacify, although it still happened.
Specs: MC 17w15a, Java 1.8.0_74 64bit, macOS 10.12.
The error is occurring because there is an extra space immediately after 'setblock'.
Meaning: (underscores are spaces) "setblock _ _ ~ _ ~1..."
It's trying to parse the second space as the x coordinate.
Can't determine if this should be a bug, just clarifying the issue.