MySQL Error #1133 - Can't find any matching row in the user table

I encountered this error using MySQL in a different context (not within phpMyAdmin). GRANT and SET PASSWORD commands failed on a particular existing user, who was listed in the mysql.user table. In my case, it was fixed by running

FLUSH PRIVILEGES;

The documentation for this command says

Reloads the privileges from the grant tables in the mysql database.

The server caches information in memory as a result of GRANT and CREATE USER statements. This memory is not released by the corresponding REVOKE and DROP USER statements, so for a server that executes many instances of the statements that cause caching, there will be an increase in memory use. This cached memory can be freed with FLUSH PRIVILEGES.

Apparently the user table cache had reached an inconsistent state, causing this weird error message. More information is available here.

This error can occur if trying to grant privileges for a non-existing user.

It is not clear to me what MySQL considers a non-existing user. But I suspect MySQL considers a user to exist if it can be found by a name (column User) and a host (column Host) in the user table.

If trying to grant privileges to a user that can be found with his name (column User) but not by his name and host (columns User and Host), and not provide a password, then the error occurs.

For example, the following statement triggers the error:

grant all privileges on mydb.* to myuser@'xxx.xxx.xxx.xxx';

This is because, with no password being specified, MySQL cannot create a new user, and thus tries to find an existing user. But no user with the name myuser and the host xxx.xxx.xxx.xxx can be found in the user table.

Whereas providing a password, allows the statement to be executed successfully:

grant all privileges on mydb.* to myuser@'xxx.xxx.xxx.xxx' identified by 'mypassword';

Make sure to reuse the same password of that user you consider exists, if that new "MySQL user" is the same "application user".

Complete the operation by flushing the privileges:

flush privileges;