Block Manager does not add or delete blocks

[Sonya*]

4.6.10 Beta 2.

I can add new blocks on the page*. But I cannot remove them. I open Block Manager and remove them. After refreshing the page, the blocks are still there. I have to remove them several times till they are really removed.

*Additionally, when I use:

{{\IPS\Output::i()->sidebar['enabled'] = FALSE;}}

in page template, I am unable to add any block to the header or footer area of this page. The blocks are there and visible, till I close Block Manager. Then they disappear. 

Probably related to this