diff --git a/src/Composer/Util/Git.php b/src/Composer/Util/Git.php
index 81c685011..27bfe766f 100644
--- a/src/Composer/Util/Git.php
+++ b/src/Composer/Util/Git.php
@@ -97,6 +97,7 @@ class Git
 
         $auth = null;
         if ($bypassSshForGitHub || 0 !== $this->process->execute($command, $ignoredOutput, $cwd)) {
+            $errorMsg = $this->process->getErrorOutput();
             // private github repository without ssh key access, try https with auth
             if (preg_match('{^git@' . self::getGitHubDomainsRegex($this->config) . ':(.+?)\.git$}i', $url, $match)
                 || preg_match('{^https?://' . self::getGitHubDomainsRegex($this->config) . '/(.*)}', $url, $match)
@@ -117,6 +118,8 @@ class Git
                     if (0 === $this->process->execute($command, $ignoredOutput, $cwd)) {
                         return;
                     }
+
+                    $errorMsg = $this->process->getErrorOutput();
                 }
             } elseif (preg_match('{^https://(bitbucket\.org)/(.*)(\.git)?$}U', $url, $match)) { //bitbucket oauth
                 $bitbucketUtil = new Bitbucket($this->io, $this->config, $this->process);
@@ -149,6 +152,8 @@ class Git
                     if (0 === $this->process->execute($command, $ignoredOutput, $cwd)) {
                         return;
                     }
+
+                    $errorMsg = $this->process->getErrorOutput();
                 } else { // Falling back to ssh
                     $sshUrl = 'git@bitbucket.org:' . $match[2] . '.git';
                     $this->io->writeError('    No bitbucket authentication configured. Falling back to ssh.');
@@ -156,6 +161,8 @@ class Git
                     if (0 === $this->process->execute($command, $ignoredOutput, $cwd)) {
                         return;
                     }
+
+                    $errorMsg = $this->process->getErrorOutput();
                 }
             } elseif (
                 preg_match('{^(git)@' . self::getGitLabDomainsRegex($this->config) . ':(.+?)\.git$}i', $url, $match)
@@ -186,6 +193,8 @@ class Git
                     if (0 === $this->process->execute($command, $ignoredOutput, $cwd)) {
                         return;
                     }
+
+                    $errorMsg = $this->process->getErrorOutput();
                 }
             } elseif ($this->isAuthenticationFailure($url, $match)) { // private non-github/gitlab/bitbucket repo that failed to authenticate
                 if (strpos($match[2], '@')) {
@@ -224,10 +233,11 @@ class Git
 
                         return;
                     }
+
+                    $errorMsg = $this->process->getErrorOutput();
                 }
             }
 
-            $errorMsg = $this->process->getErrorOutput();
             if ($initialClone) {
                 $this->filesystem->removeDirectory($origCwd);
             }
@@ -248,6 +258,8 @@ class Git
                 };
                 $this->runCommand($commandCallable, $url, $dir);
             } catch (\Exception $e) {
+                $this->io->writeError('<error>Sync mirror failed: ' . $e->getMessage() . '</error>', true, IOInterface::DEBUG);
+
                 return false;
             }