I've noticed it's only specific comments too. I was able to upvote yours fine but on another comment in a different thread I tried 5-6 times and gave up. It doesn't seem to matter if the user or comment or post is local to Kbin.social or any other instance either.

Yeah, I've tried a good ten times to upvote your comment just now, including through several refreshes and a logoff/logon cycle, and it takes me to that error page every time.

I'm genuinely a little confused because this only started happening to me a couple weeks ago, but it's been getting significantly worse lately. I know the roadmap is long, but this should probably be prioritized because it's making it feel pointless to even use the site when one of the fundamental methods of participating doesn't work a good 70% of the time.