Just to update: I was on the phone with liqpay for two days trying to figure out what is wrong. Only yesterday I have got a sensible explanation.
Apparently there was some technical issues on liqpay.com during few days and therefore my payment got stuck. Interesting enough onpay.ru wasn't accepting my payments because something was wrong with liqpay, but interkassa did accepted it, however money did not arrive to the final destination due to problems with liqpay. I have tried yesterday again using onpay.ru and all worked fine and quick as in the past.
So, my recommendation is not to use interkassa unless you have a strong reason to do so. Otherwise you may end up like me :-)