Hi there,
In specials.lisp, ECL seems to require *supports-threads-p* to be set as follows:
(eval-when (:compile-toplevel :load-toplevel :execute) (define-symbol-macro *supports-threads-p* #+:lispworks t #-:lispworks bt:*supports-threads-p*))
Apparently the symbol macro is not available when compiling the file.
Otherwise compiling the file twice works when loading.