Something must have been changed with buffers between the versions:

ConTeXt  ver: 2011.11.29 18:47 MKIV  fmt: 2011.12.31

and

ConTeXt  ver: 2012.01.09 10:08 MKIV  fmt: 2012.1.11

Formerly this was ok:

\startbuffer[abc]

whatever

\stopbuffer

Now I get: ! Missing } inserted.

Because running this in isolation goes without a problem, I suspect the change has to do with the project structure in which I use the buffer. But again, it worked in the past.