I had downvoted an answer which was relating factually wrong information.
In the meantime the posting has been edited to reflect this and now I want to remove my downvote but I can’t. Every time I try to either remove my downvote or convert it into an upvote the system pops up a dialog saying “An error has occurred – please retry your request.”
EDIT In the meantime I was able to undo my vote. However, the original problem was reproducible over a time period so I’m not sure that the issue was just a temporary glitch.