Please everyone, calm down and stop opening external tickets without explicit/expressive data.
This issue is annoying, but it is not super critical. So give our infra team some time to check out what is going on and if/how-far this is related to code/apps/plugins from us or code from upstream Discourse, in order to then forward to the Discourse team the data that is necessary (if anything is necessary at all).
If people open topics here without adding data about logs, context, configuration, etc., but only elaborations of symptoms, we would do the same. They have not received much to work on atm. More precise information is necessary to tackle this effectively and efficiently, and that is available only to our infra team and those with access to our instance(s).
So please give people time to tackle this effectively and efficiently, in an ordered way 
Feel free to use this topic to exchange and verify mitigations for the meantime.
I have seen so far clearing cache helps some, and in my case, I just set my Firefox from “strict” security settings to “standard”, and then back to “strict”, then reload → that works for some time and can be repeated. Maybe others find other ways that help them.