I am more than frustrated, that's why I am offering a €20 reward to the person that offers me a solution to my problem! (payment method: bank wire or PayPal preferrably).
Sorry to hear about your problem; I do have one comment regarding your last paragraph:
It is thoughtful of you to offer money; however, I for one would not share personal and sensitive financial information with a total stranger to give you access to my account and discourage anyone from doing so.
FWIW, we are also not allowed to post personal information on these user to user forums - it is removed very quickly.
Good luck in getting your issue resolved by one/some of our knowledgeable volunteers!
Ok, I didn't know that - then I would offer to donate €40 to a charity of the choice of the person who solved my issues. I can take screenshots to prove everything afterwards...
I have spent at least 20 hours trying to fix this problem and I'm about to throw my macbook out of the window, but I still have the feeling that it's just a small thing that I didn't see yet...
The intention I had by offering a small reward/incentive was just to attract more attention to my case. I appreciate all your help and input tons. I also can't imagine that I am the only one experiencing these kind of issues, because every program/app which I use is so standard.
Coming back to my problem: What I have tried without (success) was creating another 'Test' account. IUnfortunately, the uploads also didn't work in the safe mode. I couldn't see any particular difference to my uploads under normal settings...
It is very strange, because the upload always starts (no matter what I am trying to upload). After just a few seconds, the speed decreases drastically until the upload eventually stops. Another strange thing is that I usually can't even post links on websites (i.e. If I would like to post a Youtube song on Facebook it usually doesn't work because Facebook automatically embedds it into it. However, if you click on the small 'x' (in which the song is embedded), I can post just the link without any problems).
I also used the utility manager in order to fix the rights for my hard-drive/volume.
What I should probably also mention is the fact that after reinstalling Mountain Lion, it worked for a couple of hours. However, after I installed a very limited amount of very basic programs (I think it was just Office 2011, Google Chrome, Flash, (Java) + ?) it didn't work again...
Then the thing to do is to re-install ML again and see if it works. Assuming it does then add the programs you added one at a time giving sufficient time between each for a thorough test.
Post back when you have discovered the program that causes the problem.