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If you disable a plugin then the code of the plugin does not run. Code only does something when it runs. When the code does not run it cannot do anything.

Moreover, you have proven that the problem is unrelated to the plugin by re-enabling it and having the same issue.

When you disable a plugin its code no longer executes. This is by definition. Joomla only loads and executes plugins which are enabled. It does not even look for disabled plugins. Their code may be there, or not. It may be executable, or completely broken. It doesn't matter, because Joomla will neither load nor try to run disabled plugins. It's as if they do not exist.

That's the whole point of Joomla having the ability to disable plugins: disabling the plugin serves the same purpose as deleting it entirely, without the potential problems which could arise from deleting a plugin entirely (e.g. losing the code forever, or later restoring the wrong version of the code you deleted thereby breaking your site).

Since the code of the disabled plugin no longer executes it cannot have an effect on your site. Despite the fact that this is common sense, and quite obvious, you seem to be disputing that? I refrained from further explaining it exactly because it is obvious and explaining the obvious would make me come across like a condescending jerk. But if you insist, here it goes…

Take a USB thumb drive and put a script in it which deletes all files and folders on a hard drive. That script is, in fact, code. Copy that script to a computer but do not run it. Do the files and folders on that computer magically disappear? No, of course not! Why is that? Because the script, the code telling the computer to delete all files and folders, is not executed (run) by the Operating System, even though a hell of a lot of other code on it runs all the while. What does that tell us? The painfully obvious: computers are dumb machines which have no mind of their own, and will not do anything unless they are explicitly instructed to do so by running a set of instructions (programme). Therefore, the mere presence of code which is not being executed does not, and cannot, mean that this code has anything to do whatsoever with what the computer is doing. Your server is just a computer. It's that simple.

Now let's look at what you have explicitly stated on your own accord:

If I disable admin tools the system page becomes completely empty!

Even reactivating admin tools from phpmyadmin the system page remains completely empty!

Let me state the exact same thing in different words so it's easier to understand.

When the plugin is disabled therefore its code does not run therefore it cannot have an effect on your site you have a problem.

When the plugin is enabled therefore its code does run therefore it can have an effect on your site you still have the same problem.

The only conclusion a logical person can come to is that the plugin and its code does not and cannot play a role in your problem at all, whatsoever, since whether it is running or not makes no difference and we already know that code which does not run cannot have an effect on the rest of the site's code which does run.

This does not mean that there is no problem. I never disputed that. It only means that you are barking up the wrong tree.

The next logical step in troubleshooting this problem would be going to Global Configuration and doing two things. Set Debug Site to Yes, and set Error Reporting to Maximum. This will show you an error message in the System page; if you do not understand it, paste it here and I will understand it for you. If you receive no error message then the host is blocking that page and you need to contact the host.

My guesses are that you are running out of memory, a third party plugin experiences an error, or your host is accidentally blocking access to the System page due to an unfortunate and too generic mod_security2 rule in their Apache configuration. These guesses are not based on any information since I do not have any to go by; they are merely the three top reasons why I have seen sites "suddenly" breaking, both live sites and development sites. It is very likely that my guesses are wrong. I am a software developer, not an omniscient deity.

If you do the troubleshooting I mentioned above and see an error message, please do share it with me. I can help you understand what it means :)