diff --git a/src/Pure/Admin/build_cygwin.scala b/src/Pure/Admin/build_cygwin.scala
--- a/src/Pure/Admin/build_cygwin.scala
+++ b/src/Pure/Admin/build_cygwin.scala
@@ -1,89 +1,89 @@
 /*  Title:      Pure/Admin/build_cygwin.scala
     Author:     Makarius
 
 Produce pre-canned Cygwin distribution for Isabelle.
 */
 
 package isabelle
 
 
 object Build_Cygwin
 {
-  val default_mirror: String = "https://isabelle.sketis.net/cygwin_2020"
+  val default_mirror: String = "https://isabelle.sketis.net/cygwin_2021"
 
   val packages: List[String] =
     List("curl", "libgmp-devel", "nano", "perl", "perl-libwww-perl", "rlwrap", "unzip")
 
   def build_cygwin(progress: Progress,
     mirror: String = default_mirror,
     more_packages: List[String] = Nil)
   {
     require(Platform.is_windows)
 
     Isabelle_System.with_tmp_dir("cygwin")(tmp_dir =>
       {
         val cygwin = tmp_dir + Path.explode("cygwin")
         val cygwin_etc = cygwin + Path.explode("etc")
         val cygwin_isabelle = cygwin + Path.explode("isabelle")
         Isabelle_System.mkdirs(cygwin_isabelle)
 
         val cygwin_exe_name = mirror + "/setup-x86_64.exe"
         val cygwin_exe = cygwin_isabelle + Path.explode("cygwin.exe")
         Bytes.write(cygwin_exe,
           try { Bytes.read(Url(cygwin_exe_name)) }
           catch { case ERROR(_) => error("Failed to download " + quote(cygwin_exe_name)) })
 
         File.write(cygwin_isabelle + Path.explode("cygwin_mirror"), mirror)
 
         File.set_executable(cygwin_exe, true)
         Isabelle_System.bash(File.bash_path(cygwin_exe) + " -h </dev/null >/dev/null").check
 
         val res =
           progress.bash(
             File.bash_path(cygwin_exe) + " --site " + Bash.string(mirror) + " --no-verify" +
               " --local-package-dir 'C:\\temp'" +
               " --root " + File.bash_platform_path(cygwin) +
               " --packages " + quote((packages ::: more_packages).mkString(",")) +
               " --no-shortcuts --no-startmenu --no-desktop --quiet-mode",
             echo = true)
         if (!res.ok || !cygwin_etc.is_dir) error("Failed")
 
         for (name <- List("hosts", "protocols", "services", "networks", "passwd", "group"))
           (cygwin_etc + Path.explode(name)).file.delete
 
         (cygwin + Path.explode("Cygwin.bat")).file.delete
 
         val archive = "cygwin-" + Date.Format("uuuuMMdd")(Date.now()) + ".tar.gz"
         Isabelle_System.gnutar("-czf " + Bash.string(archive) + " cygwin", dir = tmp_dir).check
       })
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_cygwin", "produce pre-canned Cygwin distribution for Isabelle", args =>
     {
       var mirror = default_mirror
       var more_packages: List[String] = Nil
 
       val getopts =
         Getopts("""
 Usage: isabelle build_cygwin [OPTIONS]
 
   Options are:
     -R MIRROR    Cygwin mirror site (default """ + quote(default_mirror) + """)
     -p NAME      additional Cygwin package
 
   Produce pre-canned Cygwin distribution for Isabelle: this requires
   Windows administrator mode.
 """,
           "R:" -> (arg => mirror = arg),
           "p:" -> (arg => more_packages ::= arg))
 
       val more_args = getopts(args)
       if (more_args.nonEmpty) getopts.usage()
 
       build_cygwin(new Console_Progress(), mirror = mirror, more_packages = more_packages)
     })
 }