Can't create products, fatal error

My cs cart installation stopped working properly after some time.

I'm not able to create products mainly due to “add category” is bugged now.

Here is step by step guide:

  1. I create a product and enter the name
  2. Trying to chose category because product can't be saved withot one chosen.
  3. When I click “add categories” button, my browser warns me that you will be navigated away from the page.
  4. Then I see the list of categories but in a weird format text list.
  5. When I chose the categories and try to save, I get an error

    “Fatal error: Cannot use string offset as an array in /store/var/cache/templates/admin/%%E4^E42^E4252AD1%%mainbox.tpl.php on line 55”

    It worked fine for some time when I changed browser from Chrome to Firefox but now it doesn't work on all browsers.

Weird, it works fine from other PC. What could be the issue?

Clear cart and browser cache.

Sadly that didn't help. One more thing I noticed that product is being closed when I hit save. It should do that in save and close only if I recall right.