just wanted to say thanks for these posts, it solved my problem. Mine was actually a combination of two which are discussed in this post.
I droped my phone so SD card has moved a little and my media scannig was actually never done. It was always in notification area and working, so that was one, and cashe files was another one. My problem mostly manifested when I enabled mobile data (edge/3g from quick settings) because android market automaticaly tried to download some updates but having problem with cashe it always failed.
As a referial to the post that these phones are complicated..
The thing is, if you buy a computer (in a form of a small phone) it needs a little knowledge and effort for proper use. If you buy a PC and install some bad software no metter how good it is it will crash.. These are also computers (processor, GPU, VPU, RAM, ROM etc..) where a phone is just another funcitonality of it. That means you have to know at least aproximately how it works, and it's basic mechanisams or else you will either have to reduce your functionality or learn a thing or two.
If you think that it is too much for you, then you should buy some phone which isn't a real computer and settle with its functionalities.
You cant expect to just click anywhere on the screen without thinking about it and that it will just read your mind and do whatever you wanted..
More functionalities mean more effort for proper use, because more options gives you more ways to use but also to break your device. Simple as that.