A 400 Bad Request error means the web server received a request it considers malformed, so it refuses to process it. Unlike a 500 error, the problem is usually on the request side: the URL, cookies, headers or uploaded data. This guide walks through the causes in the order they are most likely, so you can fix the 400 error quickly.
Before you start
- The exact URL that shows the error and the browser you used.
- Access to your cPanel account (log in from Services in the MdmHost dashboard) if you suspect the site itself.
What causes a 400 Bad Request error
- A malformed URL – stray characters such as
%, spaces, or a broken query string. - Corrupt or oversized cookies – the most common cause when only one site fails in one browser. Servers reject requests whose headers are too large (often shown as 400 Request Header Or Cookie Too Large).
- Stale browser cache or DNS cache pointing the browser at old data.
- A file upload that is too large or sent with a broken form.
- An application bug – a plugin, API client or script building invalid requests.
How to fix the 400 error step by step
- Check the URL. Retype it rather than pasting it. Look for double
%%, unencoded spaces or characters copied from a document. - Open the page in a private window. If it loads there, the problem is your cookies or cache.
- Clear the cookies for that site only. In Chrome: click the icon left of the address → Cookies and site data → delete. Then reload.
- Clear the browser cache and, on Windows, flush the DNS cache with
ipconfig /flushdns. - Try a smaller upload. If the error appears when submitting a form or uploading, test with a small file. See upload size limit errors to raise limits.
- Disable browser extensions that rewrite headers (privacy tools, VPN add-ons).
- Check the server logs. In cPanel → Metrics → Errors look for entries at the time of the request; the message usually names the bad header or parameter. Our error and access logs guide explains where to look.
Tip: If every visitor gets a 400 error on one page, the cause is on the site (a plugin or a bad rewrite rule), not in the browser. Test with a second device to tell the difference quickly.
Fixing 400 errors on WordPress sites
On WordPress, 400 errors often come from a security or caching plugin rejecting a request, or from admin-ajax.php calls with missing parameters. Temporarily rename the plugin folder in File Manager and retest, following our plugin conflict guide.
What to check afterwards
Reload the page in a normal window, submit the form again, and confirm nothing new appears in the error log. If the error came from cookies, it may return if the site keeps writing large cookies, so report that to the developer.
FAQ
Is a 400 error my fault or the website's?
Usually it is caused by the request, so start with the browser. If it happens on every device, the website is at fault.
Does a 400 error hurt SEO?
Only if search engines receive it. Pages that return 400 to Googlebot will drop from the index, so fix site-wide 400s quickly.
What does "Request Header Or Cookie Too Large" mean?
It is a 400 variant: the cookies sent for that domain exceed the server's header limit. Clearing the site's cookies fixes it.
Next steps
- Common HTTP error codes explained
- Website down troubleshooting checklist
- Still stuck? Open a ticket with the URL and the time of the error.