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I contacted CIC about the problem, this is what I received in return (they were pretty quick to reply):

The error message you described in your email is the result of a periodic technical problem. That means that you may still be able to access IRCC's online services before the situation is resolved on a permanent basis. Please try again.

We apologize for any inconvenience that this may have caused. Rest assured that the responsible department is currently making all the necessary efforts to resolve the issue as quickly as possible.
 
I was able to log in a few hours ago. now I either get error or loop through login page and credentials page.
 
It is all the day I am having problems logging in. It is very stressfull. I cleared cache and cookies, and open with incognito mode.
Unbelievable seems from 30.06.2018 to today, the problem is still unsolved!!!
 
I am struggling with this error from past 2 days and I have to upload some documents immediately. No browser is working and all leading to 403 error. Once I was successful to hit the upload buttom, then the same error screen. Tried calling the support center and they are not taking any calls at this moment. I am not sure what to do now :-(