Common WordPress Issues

It is recommended to enable debug mode to if you are experiencing errors. To enable debug mode, login into your server (not WordPress admin backend) and locate to where your site is installed, open yoursite/wp-config.php and look for this line:

define('WP_DEBUG', false);

then set to true like this: 

define('WP_DEBUG', true);

For more information on debugging:

Here are some issues you might experience when using WordPress.

The link you followed has expired.

Usually happens when you try to install new theme/plugin which its size bigger than server configuration. 

Solution: increase upload_max_filesize and post_max_size in PHP settings.

Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 2348617 bytes)…

Happens when your site doesn’t have enough memory to perform properly, sometimes, using heavy weight plugins also leads to this issue.

Solution: increase memory_limit in PHP settings .

Parse or Syntax Error.

Occurs when adding code snippets to your site via functions.php

Solution: Login into your server via cPanel or FTP and remove added code or correct the error.

Internal Server Error or 500

This means that something went wrong but your server was unable to identify the issue. Sometimes exceeding memory limits or making changes on server configuration files can trigger this error.

Solution: Try to undo the recent changes to your server configuration files or increase memory_limit.

Error Establishing a Database Connection

This means your site can not connect to database for some reason, it could be inexistent database or database is corrupted or even database server is crashed.

Solution: you can try to fix on your own by adding this line to wp-config.php

define('WP_ALLOW_REPAIR', true);

if this doesn’t work, talk to your hosting manager, if you use UNIX based VPS, try to reload database server using these commands:

for MySQL:

systemctl reload mysql

for MariaDB:

systemctl reload mariadb

WordPress asks for FTP credentials (connection information) when installing/updating theme or plugins.

This is due to permissions restricted, WordPress asks for credentials to processs your requests like installing plugin or even deleting media content.

Solution: The easiest way to solve this problem is to add the FTP information to your wp-config.php, right below WP_DEBUG line:

define('FS_METHOD', 'direct');
define( 'FTP_USER', 'username' );
define( 'FTP_PASS', 'password' );
define( 'FTP_HOST', 'ftp.example.com:21' );

Sometimes above solution may not work as expected, if you are using dedicated UNIX based VPS try this command:

sudo chown -R www-data:www-data /var/www/your-site-directory

Read more on WordPress Codex here