Just a growing pain when there are surges of users like this. Most of instances created with less than 100 in mind but got blown up due to reddit drama and ends up receiving more than 1000 new users and 10x more than anticipated posts/interaction. Nothing new here. Just like microblog-style part of fediverse (a.ka mastodon, pleroma, calckey, etc). The devs and admins will quickly solve the problem in the coming days/weeks.
If I may suggest to click it once and wait for it to register, Lemmy server will receive the request but it will take a little time to register due to server load. I suspect multiple clicks will toggle the upvote on and off.