Just signed up to say I'm having exactly the same issue with my own server.
According to my server host's control panel, the server is on 1.0.12a, and there was a server restart around 14 hours ago which I believe was shortly after the update was released. However if I update my client to 1.0.12a I get a message when trying to log into the server that a number of microblocks are missing.
Even more strangely, my friend who is also on the server has the same issue with his client on 1.0.12a, but can log in fine if he reverts to 1.0.10b, leading me to believe that the server hasn't actually been upgraded and is still on that older version. When I try to do that on my client however, I still get the missing microblocks message.
Really confused... is there any way to confirm the modpack version on both client and server (like, in the files anywhere?) I want to A make sure my client is actually reverting (had some issues with the Technic launcher a few weeks ago where it kept forgetting the modpack installation folder... wondering if this is similar perhaps?) and B confirm whether or not the server really has been upgraded or if my user CP is lying to me lol.