Did you look at that link I provided for rebuilding the database for Entourage? I remember when I was using Entourage, I would rebuild its database occasionally.
Also, I'll ask my usual questions: 1. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You can actually do quite a lot of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with that.
Are you making backups to an external device? That task is so, so critical. If you are doing that, what software do you use for that task?
How much space is on your internal drive? When one checks the site: it indicates that Office 2008 compatible with High Sierra, but Entourage is questionable. You might want to look at these.
As long as you followed the instructions, you did not do anything wrong. But as I pointed out, there are issues trying to use Entourage and Office 2008 with High Sierra. I had been using Outlook 2016 on both of my machines, and up until late August, everything was fine. (I follow those steps religiously that I asked you about above, and make a concerted effort to keep both of my Macs 'lean, mean, and clean').
Well, all of a sudden, I could not access my EMails (on my ISP's (Comcast) server with the Mini with Outlook 2016. No matter what I did, I could not get it to work any longer. I was able to get onto comcast's site and access my EMails via my browser. But no go on the Mini.
Computer Entourage
I thus decided to move to a new EMail client/program, Thunderbird. It is free, developed by the same folks who develop Firefox, and it has had a number of positive reviews. However, due to the way Outlook 2016 'stores' EMails, I needed to purchase the excellent program LOM Extractor Pro for $49 to first 'convert' the Outlook 2016 EMails, and then I was able to import all of them into Thunderbird. I did that on both of my Macs, and have not looked back! That program has been working flawlessly for me.
So if you are not 'stuck' on Entourage, you might want to look into Thunderbird. Not sure if the EMiais stored by Entourage can be imported directly into Thunderbird or not, but it's worth a look.
If you need to stay with Entourage, what you could do is do a recovery from the last Time Machine backup that was taken prior to the issue occurring. In fact, it might be wise to wipe your hard drive clean, re-format it, do a clean, fresh installation of High Sierra, and then 'migrate'/copy needed 'things' for that Time Machine backup. By the way, I have a test 'environment' with a Samsung external SSD, using it to test Mojave and my applications. I am pleased to say that all is well, and Thunderbird works fine with Mojave (now using it with V10.14.1). Also, I ditched Word 2016 and Excel 2016 for the excellent free program/suite LibreOffice.
It works flawlessly with Mojave also.