Error when reading from it's preview cache

There will always be a previews folder. By renaming or deleting the previews folder, Lightroom is forced to create a new one. I hope that resolves your issue, and I hope Lightroom 5 continues to serve you well. However, holding onto Lightroom 5 could be holding you back. There have been several processed version updates and numerous improvements that have definitely made an improvement in what I have been able to do with many of my images. Maybe that isn't a consideration for you, but there it really is a noticeable difference that, in my opinion, is well worth the money. And you get periodic updates as part of that subscription fee. But if you are content with Lightroom 5 then so be it.

"Normally", the catalog and its associated files are stored in a Lightroom folder inside of the Pictures folder on the main hard drive on the computer. That doesn't mean that the catalog must be in that folder. That is the default folder where the installation process puts it. The previews folder we are discussing would be in that same folder. But since you renamed folders on the external drive, and that has changed what you are able to view in Lightroom, that makes me suspicious that your catalog is on the external drive. That's why I was asking.

There are two different versions of Lightroom today. You ask about a "basic" Lightroom. The version that operates similarly to what you are accustomed to is Lightroom Classic. It is part of the creative cloud program. That does not mean that the program is in the cloud. It is delivered to you via the cloud, but it is installed locally on your computer and functions much like the Lightroom 5 you have been using. It has many new functions that your version does not have and has the additional capability of sharing collections on the Web if you choose to do so. It is not possible to purchase Lightroom Classic out right. There is no version of Lightroom that can be purchased. All Adobe products, except for Photoshop Elements and Premiere Elements, are part of the creative cloud system and are only available via subscription. The basic photography plan includes Lightroom Classic and Photoshop, and is $9.99/month in the US, with a minimum one-year commitment. There are additional features that are included such as a mobile version that can be installed on your phone and other mobile devices, but I won't try to go into details on that aspect unless you are interested in learning more about that. A lot has changed, really a lot, since Lightroom 5.

Is the catalog you are using on the external hard drive?

This is the forum for Lightroom, the cloud-based Lightroom that stores images in the cloud. Adobe has confused many users by their naming convention. However, since you are using Lightroom 5, I will attempt to answer your questions. The file that has begun to cause you problems is the previews folder which has the extension .lrdata.

It is the only folder that you need to delete. It is located in the same folder that contains your catalog. After you delete (or simply rename that folder if you prefer) that folder then Lightroom will rebuild your previews folder. Adobe does not provide support for Lightroom 5 anymore. It wouldn't do you much good to call Adobe because Lightroom 5 is not a product they will offer ANY support for because it is ancient history, not under development, and no longer supported. The only place you are likely to receive support for it is on this forum. If you rename that folder rather than delete it, if the rebuilding process doesn't work as expected then you can simply rename it back to what it was previously and everything will be as it was and you can pursue some other solution. But I suspect the solution I have suggested is what is needed.