Can't create table, but table doesn't exist
InnoDB Architecture

ANALYSIS

Somehow, you lost the my_user.frm and my_user.ibd files. The data dictionary still has an entry for that table.
You cannot run DROP TABLE my_user; because mysqld looks for the my_user.frm first. Since this is no my_user.frm, the table cannot be dropped.
Although my_user.frm does not exist, you cannot run CREATE TABLE my_user ... because mysqld thinks it is OK to create the table but then defers to the storage engine. InnoDB says &quot;I already have the tablespace_id of my_user registered&quot;.

This sequence of events can be proved if you create the table using MyISAM. mysqld will allow it. Once you switch to InnoDB, it goes right back to the data dictionary, which is faulty on that one entry.
I have two suggestions
SUGGESTION #1
Don't create the table with that name anymore. Use a different table name
CREATE TABLE my_usertable (id INT AUTO_INCREMENT NOT NULL, username VARCHAR(255), group_id VARCHAR(255) DEFAULT NULL, PRIMARY KEY(id)) DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci ENGINE = InnoDB;

This will result in you changing the table name in your application code
SUGGESTION #2

mysqldump all the data, triggers and stored procedures
backup ibdata1
delete ibdata1
reload the mysqldump
Detailed steps from my StackOverflow post

I have dealt with this problem before in my post InnoDB table SELECT returns ERROR 2006 (HY000): MySQL server has gone away (after power outage)