There is a known bug in version 3.8.3 R1 where if you have Search Engine Friendly URLs turned on, it doesn't work for text pages or manufacturer pages. While they do work if the setting is turned off. The patch for this bug is below. You will want to download the patch, unzip it to your computer hard drive, then upload it to your store's root directory using the File Manager or FTP.
Download Patch Here.