I've traced back a breaking change to v3.0.9. Here's the minimal repro:

str = "\xFE\xFFColumn1,Column2\n" + "\"value1.1\",value1.2\n"
io = StringIO.new(str)
io.gets *["\n", 8192]

In v3.0.8:

io.gets *["\n", 8192]
#=>  "\xFE\xFFColumn1,Column2\n"

In v3.0.9:

     ArgumentError:
       encoding mismatch: UTF-16BE IO with UTF-8 RS

I suspect there's some kind of action for me, but this error message is a bit opaque (i.e. what is RS?).