All versions up to 2.2.1 on some servers

Replace all instances of "mysql_escape_string" with "addslashes" in initialize.php, or simply remove those function calls. Note that if you perform the latter action, you must ensure that the values you put in the "Application Name" and "Administrator Email" fields of the installation form do not contain any apostrophes (otherwise, the resulting installation will be broken).

All versions, on GoDaddy hosting

All web applications featuring human-friendly URLs (i.e. Joomla!, Drupal, Wordpress) will have this issue on GoDaddy. It is caused by the URL rewriting engine being disabled in GoDaddy's hosting environment. See the following articles on how to rectify the issue using .htaccess overrides:

• Magento: "No Input File Specified"
• GoDaddy — Enabling Joomla! Search Engine Friendly URLs

• Installed at 1.6.1 or earlier and updated to a version in the range 1.6.5 - 2.0
• Installed on any server where the PHP option allow_url_fopen is disabled

• Enable fopen_allow_url in the hosting environment (updates themselves won't work without this option; it's the crux of the remote update process)
• Set value of the column unique_id in table x2_admin to "none", i.e.

  UPDATE `x2_admin` SET `unique_id`='none'
  

  Then, go to "Updater Settings" ("General Settings" in versions earlier than 2.0), ensure "Enable Software Update Notifications" is checked, and submit the form.
• Run the update manually

• Installed at 1.6.1 or earlier and updated to a version in the range 1.6.5 - 1.6.6

Run the following SQL:

Versions earlier than 2.0:

INSERT INTO x2_fields (modelName, fieldName,  attributeLabel,  modified, custom, type,  required, readOnly, linkType, searchable, relevance) 
 VALUES ("Quote", "products",  "Products",  0,  0, "varchar", 0,  0,  NULL,  0, ""), 
        ("Quote", "existingProducts", "Existing Products", 0,  0, "varchar", 0,  0,  NULL,  0, "")

Versions 2.0 and later:

INSERT INTO x2_fields (modelName, fieldName,  attributeLabel,  modified, custom, type,  required, readOnly, linkType, searchable, relevance,isVirtual) 
 VALUES ("Quote", "products",  "Products",  0,  0, "varchar", 0,  0,  NULL,  0, "", 1),
        ("Quote", "existingProducts", "Existing Products", 0,  0, "varchar", 0,  0,  NULL,  0, "", 1)