The root cause is one of these lines (either in src/process.c or src/main.c): printf("Accessing Gembird #%d USB device %s\n", devnum, This line goes before headers and modern browsers do not recognize this as a proper HTML. Therefore they show it as a text. Workaround for the users is to run the server in a quiet mode (-q switch): E.g.: sispmctl -q -d 0 -L