I can say I have been experiencing the same issue but not to such an extent. There are days where the website doesn't confirm the vote due to a Captcha error at first (even after verification) but, in my case, attempting to register the vote again works fine. Even so, it has been taking much more time doing so, regardless of whether the failure happens or not.
If anything, I'd say it is an issue on Minecraft Server Status' end. To further prove this, one could try voting for another server there, for science.
DISCLAIMER: In no way I am trying to encourage other players to vote for other servers for the purpose of advertising.