Nobody can tell you anything without exact system info or other technical details like your actual preview settings. for all we know you simply run out of memory, but if that isn't the case, then something else must be going on, which is impossible to tell without more info.
I had thought it could be related to memory, but then again I've used AE for other complex projects and never had an issue. Also, interestingly, the info shown under the About After Effects command states that there is 16% used of the 8 GB of memory.
Anyway, here are the specs.
After Effects CC 2017.1 Release (Version 18.104.22.168)
iMac (27-inch, Mid 2011)
Processor: 3.4 GHz Intel Core i7
Memory: 8 GB 1333 MHz DDR3
Graphics Card: AMD Radeon HD 6970M 2048 MB
Couple of things.
First, the "usage" doesn't count what is being cached into RAM for your previews. (Confusing, I know.)
Second, you should consider updating. There are a lot of bug fixes in the 14.1.0 update.
Third, it simply could be that previews used less RAM in the projects you've done before. Perhaps there were less data requirements, perhaps you were previewing smaller images or at a smaller resolution, perhaps any number of things. But 8 GB of RAM is the bare minimum to use AE. I'm more surprised that you had no issues before than that you are having issues now. I would be extremely surprised to see you get 30 seconds, much less two minutes of a full HD video at full res!
With as little RAM as you have, it's best to preview at smaller resolutions - if you MUST see your whole comp play back. But there isn't usually a need to see the whole thing while you're working. Usually I work in bits at a time and I have a machine with over 100 GB of RAM, so that's not even an issue!