app store do not work

Hello, people! Please help me!

I have iMac late 2013 (3,5 GHz i7, 32 GB 1600 MHz DDR3, Yosemite 10.10.2)

App Store doesn't work on my iMac for almost 2 months. This problem began before Yosemite update, at that time App Store was working with some difficulties but I was able to update to Yosemite - I thoght that this update will help me. But it didn't help at all! After updating system App Store does not work at all. App Store keeps showing the update sign though (number in a red circle) but obviously doesnt show any updates. Whenever I open it, it just shows the grey page, not detecting any problems. The only way I can get anything from my App Store is by clicking on Store>View my account. After waiting for some time it shows me the login window and after I log in again after several minutes, information about my account opens in safe mode only. And I can never open any other windows in App Store.

I tried:

- to create new apple ID

- to delete Little Snitch

- to work with certeficates (i saw some video where the guy explains that App Store can be not working if some certifacates have "+" icon on)

- to reboot NVRAM (after this step iTunes started working again but very slow. Before this it wasnt working exactly same way as my App Store)

- and finally to reinstal OS X (to Yosemite)

I dont know what else to try and what could possibly cause this mysterious problem! Please help!!!

P.S. my other Apple devices are working properly with the same Apple ID, which includes iPhone and Macbook Air. And only my very expensive iMac doesnt want to work anymore 😟