Having done some limited investigation I can reproduce a similar fault for about half of the time. It seems to be linked to private tabs on FF and Brave browsers - but I am still not certain of the exact mechanism. If you use an newly opened private tab for voting it is possible that there is no reskey value transferred to/from that private tab - which it can be argued is exactly the function of private tabs. Hence at some point you can be using a tab that doesn't have a reskey.
However, this doesn't happen every time so I am looking at other actions that might be taking place simultaneously.
Can you tell me which browser you use, please?
Firefox - and it's not outside the realms of possibility that I was using a private tab at the time.
My experience (see above) was also with Firefox, though in my case it was not a private tab: just trying to vote before logging in!