Unable to login to Concrete 5.6.3.3 after PHP version change

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Hi,

I am wondering if someone can help. I am having some issues with my Concrete5 site and after contacting hosting support they suggested to update the PHP version from 5.4 to 5.5. The issues seem to go away after change but now I am unable to login to my site to edit it or change settings. When I go to login page and type in my username and password and after some time I am redirected to main page (Chrome browser) or get error message 'Your browser's cookie functionality is turned off. Please turn it on.' (Firefox and Internet Explorer).

If I change the PHP version to 5.4 I can login fine but the issues I was having are still there (often errors '500 Internal Server Error').

Do you know how to fix the login issue after updating the PHP version?

I am using Concrete 5.6.3.3 with few add-ons (two main are Bootstrap Buttons, Mailing List and Power Slider Lite).