Hi Sammy2 and James
Thank you very much for your replies.
I had a few more issues on that site, not just with editor so in the end I contacted hosting company, as whatever I tried didn’t work (disabling plugins etc.) and they told me that: “We experienced a DDos attack against our servers IP address, and have placed the server under DDos protection to protect our customers. This is why it is giving that error.”
Today when I logged in everything worked as it should. So I will mark this post as resolved.
I hope you Sammy2 will get your issue resolved soon as well.