As reported at Enwiki's VPT (permalink)

I was trying to log out one day but the step was slow. It said, "You are being logged out, please wait". Then it said, "Cannot log out now: http". Eventually I was logged out.
Why so long? Does it mean that I must have been using a slow network? If I close my browser while waiting, is that good enough?

In discussion in IRC, a dev noted that:

• the relevant code is at https://github.com/wikimedia/mediawiki/blob/master/resources/src/mediawiki.page.ready.js#L57-L77
• specifically from the javascript magic that turns a click on the <a/> inside #pt-logout into a javascript POST request
• it sounds to me like the api.postWithToken() javascript call failed
• @Ladsgroup wrote the js code there https://github.com/wikimedia/mediawiki/commit/8f033911030d26759c327145403f91ac6b1c5e66

I'm filing a task

• in case it's a security problem
• in case it comes up again
• in case the error message can be improved/clarified