Topic
Fixes for the errors that take a WordPress site down.
Troubleshooting
The zip you uploaded is not a theme, or the theme is inside it one level too deep. Marketplace downloads bundle the installable zip with documentation, and uploading the bundle is the cause nine times out of ten.
Troubleshooting
Current WordPress prints "Sorry, you are not allowed to upload this file type"; the older wording still fills tutorials and old installs. Both come from the same check, and the fix depends on whether the type is disallowed or the file is not what its extension says.
Troubleshooting
The limit is PHP's, not WordPress's, and raising it in wp-config.php cannot work because the directive is read before any PHP script runs. Where it can be raised depends on the server, and two other limits sit behind it.
Troubleshooting
WordPress tried to write a folder and the server said no. The message does not say which folder or why, and the three usual reasons — ownership, a full disk, a missing upgrade directory — each need a different fix.
Troubleshooting
WordPress checked a capability and the account did not have it. After a migration the usual reason is not the role at all — it is that the table prefix changed and the row describing the account's role no longer matches.
Troubleshooting
The number is specific and useful: 28 is always a timeout, never a refusal or a certificate problem. What matters next is which request timed out, because a loopback to your own site and a call to a payment provider need opposite fixes.
Troubleshooting
The installer refused because a directory with that name is already there, usually left by an install that failed partway. The folder is the evidence, and deleting the wrong one removes a working plugin.
Troubleshooting
The editor asked the REST API for JSON and got something else back. Whatever that something is — an HTML error page, a redirect, or a stray warning — is the actual fault, and it is visible in one browser tab.
Troubleshooting
The message is deliberately vague because it is shown to visitors. The actual error is in the log, and recovery mode will email you the file and line that failed.
Troubleshooting
A blank screen, an endless spinner, or a page that will not save almost always traces back to a blocked REST API request or a JavaScript error. Work through causes in order of likelihood rather than guessing.
Troubleshooting
Removing malware is the easy half. The half people skip is proving it is gone, and every scanner type is blind to something the others catch.
Website Tools
Putting a proxy in front of WordPress solves real problems and introduces a specific set of new ones — redirect loops, mixed content, cached logged-in pages and a visitor IP that is no longer the visitor.