On Windows 10, ABCL 1.4.0 sys:run-program first issues an error.  

With abcl --no-init, the first call goes like this:

(sys:run-program "where.exe" '("mvn.bat"))

Error message:

...

 48: (JCLASS "java.lang.UNIXProcess")

 49: (SYSTEM::GET-FASL-FUNCTION #<org.armedbear.lisp.FaslClassLoader org.armedbear.lisp.FaslClassLoad.... {445C74}> 48)

 50: (SYSTEM:INIT-FASL :VERSION 42)

 51: (SYSTEM::%EVAL (SYSTEM:RUN-PROGRAM "where.exe" '("mvn.bat")))

Repeating the command the second time, and parsing the output (as done in abcl-asdf:find-mvn) gives the expected result.

Mirko