There is a known bug in 3.8.3 R1 when you have "Show nested categories products on a parent category page?" disabled. If you click into a category, you will get the following error:
Can not execute database query.
Details:
Error code: 42S22
Error info: SQLSTATE42S22: Column not found: 1054 Unknown column 'pc.pid' in 'on clause'
Query:
SELECT DISTINCT p.pid FROM products AS p INNER JOIN products_categories AS oc ON pc.pid = p.pid AND pc.cid=3 INNER JOIN catalog AS c ON p.cid=c.cid AND c.is_visible="Yes" WHERE p.is_visible="Yes" AND p.product_id <> "gift_certificate" AND ( – No stock control, pull the data p.inventory_control = "No" OR – Has any stock at all p.stock > 0 OR – Has Display Out Of Stock rule p.inventory_rule = "OutOfStock" OR (p.inventory_control = "AttrRuleInc" AND p.stock >= 0) )
The patch for this bug can be found here. Upload this file to content/classes using your File Manager or FTP.