Mildew
Before I jump into "I told you so" mode, I will tell you a few things:
If you were looking for a good "excuse" to replace the machine, then this is obviously it. Go and buy another machine now if you don't want to do anything.
If you are prepared to spend some time working on the machine, you might get lucky and be able to prelong the service life of the machine significantly:
<ul>
<li>You may have to disassemble the machine, but this lets you completely clear out any muck, and let you see the results of your washing habits (Interesting on any machine really, as it shows whether Hot/Warm washing might actually work)</li>
<li>Retrieve lost socks</li>
<li>Replace worn Spiders/Bearings/Pulleys and so on</li>
</ul>
Or you can try really hard, run your water tap HOT at the laundry (or nearest point) then absolutely PILE UP with washing machine cleaner (I know I'll be flamed for this) + Epsom Salts/Citric Acid or Bleach into the machine, WITH a decent sized load of rags, on the hottest cycle. Turn off the machine overnight, then let it complete that cycle when it is powered on (even if it drains) and run another cycle without turning it off.
Run this a few times and your machine should be clean(er) than it was.
But as everyone else is saying, you are better off following the old adage "Prevention is better than the cure." Look how much you'll spend on a new washer now, or on washing machine cleaner or your own time trying to repair the machine, when its service life could've been prolonged with some simple additional care, which would be running fairly frequent Warm or Hot washes and avoiding liquid detergents and fabric softener overdosage, not "just" leaving the door ajar after usage.
I can't judge how clean your laundry is from this cold-water stuff, but I do know that your mildew/mould issue is related to cold water washing and the buildup of fabric softener within the machine. Remember, as there is no hot water being used to suspend the oils and greases from the softener in the water, the oily stuff from your clothes (which is body oils AND softener) is being swished around in the machine, building up and resisting flushes with hot water every so often, and your clothes will have built up lots of this fabric softener "gloop" in them from your washing habits.
I don't say that from scepticism, but from actual experience. I tried washing my dark items at 30ºC few quite a while with some softener, and quickly noticed that the detergent was largely ineffective in the wash cycle. So next time, I ran those items on 50ºC and there was no softener "residue" evident in the wash, water was a little bubbly and murky, not clear and barely bubbly like before.
If you don't believe that, get some cooking oil, put it into a large (clear) bowl of cold water and a little detergent. Try that again with Hot water (as in VERY hot water) and see if you can notice a difference. If I remember correctly, the oil droplets will shrink and be held in the water, not just float on the top or remain solid. The very same thing happens in your washing machine, on a slightly different scale.
So really, I stand with the others in saying that your washing habits are likely the cause of this, but I really don't want to enter into another Cold vs. Hot water wash debate. Like they say, the person that can't use their tools properly will blame them when something goes wrong. The machine is only as good as it's user!
Please, for goodness sake, consider the advice we are giving you and try to be open minded about this. You cannot deny what we are saying, as you have not yet tried it. Remember just weeks ago you were telling us how your machine was "mould/mildew free," and "had no issues" despite the fact you washed/rinsed in cold water. Perhaps you have hidden this issue from us all this time to make cold water look "better?" I don't know. I just beg of you to consider what we are trying to tell you. Don't ask for advice, then discount everyone because you don't like the idea. That is "pig-headed," as member "ronhic" put it.