Forum Bug - can't subscribe to topic

When i am trying to subscribe to topic using "Gearbox" on right and "Subscribe to this thread" i get 404 error.

Subscription using radio input when writing post, seems to work fine.

PS I am not sure if it is right subforum to submit it, but i can't write posts in News & Disscusion

Thread about my project that is using CodeIgniter:

Topic: Mysticat.pet - online game themed around cats