Why has my slow cooker gone mouldy?

And before all the trolls start...it is THOROUGHLY cleaned after every use!!!

We got it a year ago as a wedding gift but the last two times I have got it out to use it it has been really smelly and there has been a powdery white mould in the ceramic crock pot. I scoured and disinfected it and it seemed fine. We used it, no one died, it was thoroughly washed in boiling water and totally dried before it was put away again. But I just got it out to use again and it really stinks and the mould is back but only on the outside of the crock pot this time.

I did notice, just after we used it the first time, that the two screws in the metal heating base were covered in a sticky black goo even though no food/liquid was in that part and now they look a bit rusty. Is there a fault with it that could be causing all this? If so, can I still return it even though I forgot to submit the guarantee registration card?