Why does WordPress say 'Publishing failed. You are probably offline.'?

The block editor saves through a background request to your site's REST API, and that request got no readable reply. A browser extension blocked it, a firewall or security plugin refused it, the site address points to http while you edit on https, or the server crashed or timed out during the save.

How do I find what is blocking the WordPress editor save?

Open the browser's developer tools, Network tab, and press Update. The failed request to /wp-json/wp/v2/posts/ shows the cause: failed or blocked means the browser or an extension, 403 means a firewall, 500 or 504 means a plugin error or time-out.

Will installing the Classic Editor fix 'Updating failed'?

It can hide it, because the Classic Editor saves another way, but the block on the REST API stays and breaks other tools. Fix the cause: allow the request in the extension or firewall, correct the site address, or fix the crashing plugin.

More fixes

"Publishing failed. You are probably offline." in WordPress

You press Publish or Update in the block editor and a red bar says "Publishing failed. You are probably offline." (or "Updating failed"). Your internet is usually fine. The editor saves by sending the post to your own site in the background, and that request came back with no answer it could read: something blocked it, cut it off, or sent it to the wrong address. Your text is still in the open tab, so do not close it yet.

What the owner sees

harborbakery.example/wp-admin/post.php?post=412&action=edit

What the checks found

First, keep your work

Before anything else, select all the text in the post (click in it, Ctrl+A twice, Ctrl+C) and paste it somewhere safe. The editor also keeps a local backup in the browser and offers "The backup of this post in your browser is different" next time, but a copy you made yourself is certain.

Find out which case it is (five minutes)

Open the browser's developer tools on the editor (F12, or right-click, Inspect), choose the Network tab, and press Update again. Find the red request to /wp-json/wp/v2/posts/ (or ?rest_route=). Its status is the answer:
- (failed), (blocked) or net::ERR_: the browser never got a reply. Go to step 2.
- 403: a firewall or security plugin refused it. Go to step 3.
- 500, 502, 503, 504 or a long wait then failure: the server broke or ran out of time. Go to step 4.
Try a private window with no extensions, or another browser. If it saves there, an ad blocker or privacy extension is blocking the request. If the request's address starts with http:// while the editor is on https://, the site's own address settings are wrong (Settings, General) and the browser blocks the mixed request.
Does it fail only on some posts? Server firewalls (ModSecurity on many shared hosts, or a security plugin) block saves whose text looks like code: a pasted script, an iframe, words like SELECT ... FROM. Save a short test post. If that works, remove the last pasted block and try again. The firewall log or the host can name the rule.
Is it a long post, or is the whole admin slow? A save that takes longer than the host's time limit is cut off and looks like "offline". Tools, Site Health shows REST API and loopback problems; the server's error log shows a PHP fatal error if a plugin crashes during the save.
Behind Cloudflare or another proxy? A "bot fight" or WAF rule can challenge the editor's background request. Its Security Events log will show the blocked /wp-json/ request.

The fixes, by case

An extension: allow your own site in the ad blocker or privacy extension, or turn it off on /wp-admin/.
Wrong site address: in Settings, General, make both addresses the https:// one you actually use (if they are greyed out, they are set in wp-config.php; change them there).
A firewall rule: allow the rule for /wp-json/wp/v2/ requests from logged-in editors in the security plugin, or ask the host to whitelist that one ModSecurity rule ID for your site. Paste embed code through a proper embed block instead of raw HTML where you can.
A plugin that turns off the REST API: some "disable REST API" or hardening settings also lock out the editor. Allow it for logged-in users, or switch the plugin's setting off.
A crash or time-out: the error log names the plugin; update or replace it. If the server is just slow, ask the host to raise the time limit or fix the slow plugin first.
A proxy rule: skip the challenge for /wp-admin/* and /wp-json/* when the request carries your login cookie.
Test: reload the editor, make a small change, Update, and open the post in a new tab to see the change is really saved.

What you see, what it means, what it needs

What you see	What it means	Usual fix
Saves in a private window, fails in your normal one	An extension blocks the save request	Allow your site in the extension
Network tab shows a 403 on /wp-json/	A firewall or security plugin refused it	Allow the rule for the editor, or ask the host
Only posts with pasted code or embeds fail	A firewall pattern matches the text	Use an embed block, or whitelist the rule
Request goes to http:// from an https:// editor	Site address is set to the old http one	Fix both addresses in Settings, General
500/504, or long wait then failure	A plugin crashes or the save times out	Read the error log, fix the plugin, raise the limit

What not to do

Do not close the tab or reload before copying your text.
Do not install the Classic Editor as the fix. It saves another way and may hide the problem, but the same block will hit the next tool that uses the REST API, including site checks and app connections.
Do not turn the whole firewall off. Allow the one rule for logged-in editors.

Most of these are one extension setting, one address or one firewall rule. If every save fails and you cannot find what is blocking it, that is what the box below is for. Related: "The response is not a valid JSON response", cURL error 28 and REST API errors in Site Health and "Are you sure you want to do this?".