I just had the same bug with flightgear as Ludo.

Running `guix build flightgear', guix showed it was downloading substitute information, and then errored with

guix build: error: integer expected from stream

Possibly helpful strace info:

write(14, "\36\0\0\0\0\0\0\0\3\0\0\0\0\0\0\0?\0\0\0\0\0\0\0/gnu/store/ssdka48kwx9f2z54j63mjxvn6bg502mm-flightgear-2018.3.5\0@\0\0\0\0\0\0\0/gnu/store/pr6xy604hg7yhcinnlymrkc958kl0bnn-openscenegraph-3.4.1<\0\0\0\0\0\0\0/gnu/store/g8g4paw3q3z8hkl441ddhdn0in54gi3f-simgear-2018.3.5\0\0\0\0", 232) = 232
read(14, "ptxc\0\0\0\0", 8)             = 8
read(14, "\34\0\0\0\0\0\0\0", 8)        = 8
read(14, "integer expected from stream\0\0\0\0", 32) = 32
read(14, "\1", 1)                       = 1
read(14, "\0\0\0\0\0\0\0", 7)           = 7
close(14)