I get this error nearly every time I post in this Flarum forum, whether a new discussion or a reply. Each time a red error box pops up saying "Oops" and asking me to refresh. I don't refresh, I just close the box and hit the Reply button again and it works. I don't know what else to give you, but probably needs fixed somewhere.
I also get this behavior in our installation too, but much less frequently, and my beta testers report seeing it too.
EDIT: It just did it as I posted this reply.