Patch to see what is causing the "URL key for specified store already exists" error when saving entities in Magento 2. This patch should only be temporarily applied and needs to be applied to the vendor/magento/module-url-rewrite directory.

show-detailed-error-when-duplicate-url-key.patch

From 3523dd79d949f93e31fc675b190f17baf66f36c3 Mon Sep 17 00:00:00 2001
From: Erik Hansen <erikpallhansen@gmail.com>
Date: Wed, 1 Feb 2017 13:20:02 -0600
Subject: [PATCH] Edit the 'URL key for specified store already exists' message

---
 Model/Storage/AbstractStorage.php | 5 ++++-
 Model/Storage/DbStorage.php       | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Model/Storage/AbstractStorage.php b/Model/Storage/AbstractStorage.php
index 9a57d29..dd58f4d 100644
--- a/Model/Storage/AbstractStorage.php
+++ b/Model/Storage/AbstractStorage.php
@@ -83,7 +83,10 @@ abstract class AbstractStorage implements StorageInterface
             $this->doReplace($urls);
         } catch (\Magento\Framework\Exception\AlreadyExistsException $e) {
             throw new \Magento\Framework\Exception\AlreadyExistsException(
-                __('URL key for specified store already exists.')
+                // BEGIN EDIT
+                // To assist in troubleshooting "URL key for specified store already exists", show first 2000 characters of exception message
+                __('URL key for specified store already exists.' . ' - ' . substr($e->getMessage(), 0, 2000))
+                // END EDIT
             );
         }
     }
diff --git a/Model/Storage/DbStorage.php b/Model/Storage/DbStorage.php
index 9ef1364..2dd57a7 100644
--- a/Model/Storage/DbStorage.php
+++ b/Model/Storage/DbStorage.php
@@ -114,7 +114,10 @@ class DbStorage extends AbstractStorage
                 && preg_match('#SQLSTATE\[23000\]: [^:]+: 1062[^\d]#', $e->getMessage())
             ) {
                 throw new \Magento\Framework\Exception\AlreadyExistsException(
-                    __('URL key for specified store already exists.')
+                    // BEGIN EDIT
+                    // To assist in troubleshooting "URL key for specified store already exists", show first 2000 characters of exception message
+                    __('URL key for specified store already exists.' . ' - ' . substr($e->getMessage(), 0, 2000))
+                    // END EDIT
                 );
             }
             throw $e;
-- 
2.5.3