Forever pending request fails to load "All Properties" mode while editing page

Hi all,

I'm experiencing issue when trying to edit existing page's properties. Basically "episerver/shell/Stores/metadata/EPiServer.Core.ContentData?modelAccessor={"contentLink":"3405081_3570991"}&dojo.preventCache=1573038813653" request is forever pending and prevent's "All Properties" screen to load. There is no response to that request. Preview mode works as well as I'm able to access that page as visitor/user.

Also I'm not able to create new pages of this problematic type.

Strangely it works for around 1 hour after "Recycling Pool" and then it happens.

Recently we have added a bunch of new properties to that page type:

[SelectOne(SelectionFactoryType = typeof(CustomSelectionFactory))]

And hid one property like this:

[ScaffoldColumn(false)]

Any pointers very much appreciated!