Assuming that your plaster walls were fully cured (not just dry) before painting: The problem is that you don't seal plaster unless you don't plan to paint. If moisture is a serious issue for you and you want to, you can seal it and then add a plaster skim, then paint that, but not directly paint the sealer. You do NOT NEED TO SEAL plaster walls though. Professional plasterers use a bonding agent between layers of plaster which is just a less diluted PVA usually -- as you know, it is a sealer as well as bonding agent, so moisture shouldn't be a problem except in old walls where the materials are simply old and failing.... PVA slows down the curing process of plaster enormously (because it's a sealer and prevents moisture from leaving the plaster), which is why it can take so long for plaster walls to cure. I don't know how or why the idea of sealing right before painting is getting passed around, but it is. It is a bad bad idea and, while I've heard of a few people doing it and having it be okay, I've heard many more cases of failing paint just like this.
The paint has trouble gripping sealer and so it runs off (patchy or beading paint) or flakes off later. The cracking paint is early stages of flaking (new paint somewhat lifted the base coat off of whatever grip it had found on the sealer -- it will fail eventually). To fix this, the paint just needs a surface it can grip.
You need to either 1) get your walls entirely scraped of loose paint, entirely sanded, cleaned up thoroughly, put a bonding agent over that (diluted PVA, for ex), and then skim over in plaster, let it cure, then prime with diluted emulsion paint (a couple of times), and then really paint over that, OR 2) scrape/sand the entire walls smooth and flake-free, clean up thoroughly, and just prime/paint over that, accepting that the walls will only be as smooth as you sanded them (which will not be perfectly smooth). Make sure that corners and edges get as much attention in the sanding as the rest of the wall.