• Installed at 1.6.1 or earlier and updated to a version in the range 1.6.5 - 2.0
• Installed at 2.0 - 2.1 without "Enable Software Update Notifications" in the install process
• 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)