I use flexible SSL and VK CallBack API, which sends requests to my server. Sometimes (not regulary), in the query log in vk callback api dashboard i see Error: SSL connect error, why this can happen? I am confused that this happens SOMETIMES, not on every request.
First you need to check your server logs for the exact timestamp of the error and probably you will find nothing.
I guess sometimes their request gets time-out or blocked before reaching to your server. In that scenario CF would be in charge of such a blockage. Ask for their list of IPs and whitelist them in your CF panel.