I can’t access wp-admin… setcookie(): “path” option cannot contain

There are a number of ways your cookie path could have illegal characters. Something could be wrong with your site’s home URL setting. Try carefully re-typing the setting’s value. Do not copy/paste it, actually type it in. Be sure to not include any trailing slash, but do include the “https://”. Without backend access, you must do this using the phpMYAdmin app, usually accessed through your hosting account. Navigate to your WP DB and open the options table for browsing. Find the “home” setting and edit it there. Do not change anything else.

You may have code defining an alternative value for the COOKIEPATH constant. On most sites it’s simply “/” (without the quotes) or the subdirectory path to the WP installation. There could be code setting it to some other value. The usual place for such code is in wp-config.php. Look for any code that defines COOKIEPATH. Such code will override the WP default. The file’s content’s can be examined via your hosting file manager, or you could download it to your local computer via FTP.

There could be a plugin or your theme altering the default value. This can be done by filtering the “home” option when it’s retrieved from the DB. As a test, try deactivating all plugins and switching to a default Twenty* theme. To deactivate plugins without backend access, rename the /plugins/ directory name via FTP or your hosting file manager. Also rename your current theme’s directory under /themes/. This will force WP to use a different theme. If any plugin or your theme is the cause, you should be able to now access the backend without error.

If you regain access, restore the proper directory names. Activate the desired theme and plugins, one at a time, until the error returns. The last activated module will be the cause.

Defining WP_HOME is a reasonable solution. Very strange that the entry has gone missing.

FWIW, COOKIEPATH is defined using the home option value in /wp-includes/default-constants.php. Any code that defines this before default-constants.php executes will take precedence.

If you’re at all interested in regaining normal functionality instead of relying upon wp-config.php code, you could manually enter the option yourself via phpMyAdmin. The ID value doesn’t matter much as long as it’s not already in use. Use the table’s current AUTO_INCREMENT value as the ID, then increase AUTO_INCREMENT accordingly. The field’s autoload value should be on or yes. You should then be able comment out your wp-config.php code and still access the back end.