Perhaps a different write-char call is involved.
Yes, you are right. I was mistaken.
The error is raised in the write-http-response procedure in runtime/http-io.scm. The error is caused by the use of newline to write to a binary port. newline can only write to a textual port.
Fix:
--- src/runtime/http-io.scm.orig
+++ src/runtime/http-io.scm
@@ -188,7 +188,8 @@
(write-ascii (write-to-string (http-response-status response)) port)
(write-u8 (char->integer #\space) port)
(write-ascii (http-response-reason response) port)
- (newline port)
+ (write-u8 (char->integer #\return) port)
+ (write-u8 (char->integer #\newline) port)
(write-http-headers (http-response-headers response) port)))
(write-bytevector (http-response-body response) port)
(flush-output-port port))