I know that there is a lot of discussion on DRM, Steam, Intellectual Property, etc. going on GOG forums, but I don't think there is any separate thread with those specific questions:
1) Is downloading and using a crack for a game illegal?
2) Then what about uploading it to a website?
3) And finally: what if someone learned how to do this on his own and cracked the game only on his machine without sharing it with anyone?

No because you modify the original game code. But I'm not a lawyer.

Uploading will probably get you in trouble.

Doing it for personal use and ONLY personal use is sort of a gray area at the moment. If it has no online component, who the hell is gonna find out anyways?

Really, depends on local laws.

1) You mean the case when EULA (or something) specifies that one is not allowed to modify the original gaming code (or rather binary files)?
2) Then what about mods?

It depends on where you live. Some countries have little regard for copyright at all, others have imposed pretty harsh restrictions. For a reliable answer, you really need to ask a lawyer in your country, and preferably not just any layer, but one who has specialized in the subject and kept up to date.

If you just want a general rule of thumb, you can go with "probably not legal, but probably too small an offense to be followed up on".

1 & 2 is probably illegal, depending on local legislations and the license the software is released under

3 falls under the highly legal standard of "It is only illegal if you get caught"

Yes and no.

In some countries (namely the US, UK and Germany among others) the use, download and upload of any measures to circumvent copy protection is illegal.

However: if the circumvention of this copy protection is necessary to exploit the rights that you are entitled to (so, if you need to crack the copy protection to play your legally acquired game), then it is legal. I had to do this recently to get Micro Machines V4 running on my Windows 7 64-bit machine, because the StarForce copy protection included does not work on Windows 7. If you're uploading it although there is no legitimate need to circumvent the copy protection as an original copy should work without any problems, then it is outright illegal and there is no legal defence.

If you're uploading it as a "service" to get a game working with copy protection that does not work on modern PCs, however, this is a grey area. Technically it is illegal, because you are violating copyright. While you are entitled to use a crack or create one for your own use to get the game working, you effectively have no entitlement to help others, if that makes any sense.

In reality, however, most publishers are fairly reasonable about this. I've not heard anything about any objections being raised to cubanraul's use of a NoCD crack for his Blood Omen fixes, for example. Filing suit against someone who provides an honest service to genuine buyers is a big PR faux pas. That's the crux. Not many people can do it, which is why this uploading is necessary when the copy protection causes problems. If cracking were easy, there would be no need for copy protection systems, as every Joe could simply crack his own games.

Well, my impression is that in my country (Poland) no one really knows if it's legal or not and there seem to be no law for this either.
But we know that law is not always fair / sane / healthy / reasonable. What if we could try to work out some kind of understanding of the problem? Any ideas? For example, can cracking of a game be compared to removing pages in a book or crossing some words out and making some notes, etc. ? Or maybe it's more like removing the speed limit from the vehicle - a limit imposed by the manufacturer..?
What moral consequences can stop me from modifying the game the way I want it?

Afaik in germany even for personal use it is forbidden to circumvent any copy protection, aka cracking something.

In Japan,to download and use and personal transfer are legal.
If you open those file or selling device publicly,you are committing illegal behavior,but until copyright holder sue it,harm shall not occur,because of civil affair.

Essentially you are bypassing the games security which has been incorporated to stop piracy so therefore frowned upon by developers. It is reverse engineering in a way.

With a fixed exe file you cannot get online if it is a multiplayer game and would most likely get you banned from whichever service. Also if you say have a Steam game that opens the Steam client when launched (All Steam games do regardless of age) you could find yourself and all your games gone for good if you get caught. All it would take is a slip up and start Steam with a fixed exe on one of the games and it would flag up. So in a scenario like this you delete the exe file before starting the service and it would just download the relevant file again.

But then you have if its a single player game and what you do behind your closed doors as long as it isnt shared with the world should really be ok. I would strengthen this by actually owning the origional game and registering the key or whatever DRM they use before doing anything that say bypasses a disc check. This enforces the fact that you actually bought the origional game and are supporting the developer.

Even the whole for own personal use is a grey area in most areas round the world. Some of these fixed exe files actually get games working that are years old and wouldnt run or recognise your origional disc.

Technically speaking, yes, it is illegal to circumvent the game's copy protection (in many countries, and as far as I know, Sweden is one of them).
Though in reality, I don't think anyone has got into any trouble for cracking a game that they themselves legally own (something that I always do when I'm faced with a starforce protected game, because starforce makes my computer unstable).

Any legislation in this respect is anchored in the much broader and fundamental principle of "Verbotene Eigenmacht" in German. It's a weird concept and absurdly difficult to translate (LEO gives "trespass" and "taking the law into one's own hands", which are both patently wrong).

German law differentiates between "possession" (Besitz) and "ownership" (Eigentum). Just because you possess something in German law does not necessarily mean you own it, which is basically the very core of what happens when you buy a game.

The idea behind Verbotene Eigenmacht is that no person is permitted by default to resort to self-help if they are being deprived of those rights, but the German Civil Code gives a long, long list of exceptions to this rule.

Article 859 for example says that that possessors are permitted to use "force" (Gewalt) in resisting the unlawful appropriation of possessions. The word "Gewalt" in the legal sense does not just mean violence or force, but any action that is likely to influence, change, damage or be resisted by the other party. So, for example, if someone stole my phone and put it on a table, I'm allowed to apply "Gewalt" by going to that table and reclaiming it.

Likewise, I have legal possession of a licence and the owner is not entitled to deprive me of this licence without good cause (assuming the EULA does not grant the owner to withdraw it at any time). So if they shut down DRM servers, I am entitled to use "Gewalt" (i.e. by destroying the copy protection system) to enforce my possession rights. My possession of the licence could only be revoked, for example, by my own culpable behaviour. and seeing as I am not culpable, the licence is still in force.

Ain't law wonderful?

Afaik you're never allowed to break any protection. You might be entitled to return it to the shop and (ask for a compatible replacement or) request your money back though.