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Maybe you're still seeing the thumbnails for the old NAS - does it look greyed-out in the tree?
ThumbsPlus has to match its thumbnail records in the database to the actual files in your system. With regular harddrives, it identifies them by their volume label. With network drives, it usually matches them by their share name (UNC). Drive lettters are not used. If your new NAS drive has a different server name, TP probably won't match your existing thumbnail records to the files on the new NAS (but see: Options | Preferences | Thumbnails to see how it performs the match on your system).
For this reason of matching files to their corresponding thumbnailrecords, it's important that you're consistent in how you approach your drive. In the tree, you may see it as a regular drive due to its mapping to a drive letter, but you'll also see it in the network branch. To ThumbsPlus, those are two different drives and when you thumbnail one, those thumbnails won't show when you look in the other.
(you can disable the display of certain drives, or the network branch, in the tree panel using Options | Preferences | Disks & Folders).
This is not a direct answer to your question but hopefully it will help you to find the solution.