Error: "Could not complete the operation."

[Gatis Tomsons]

Hi, I just wanted to update "Maintenance window" settings and after pressing save it alerts "Could not complete the operation." without any other explanation. I had a similar error on some other instance after restarting it I was able to edit settings, but this is a production instance and restart is not a good option. Why this error shows up and what really does it mean?

Thanks in advance.