PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry

Hi, I've got an error in the registration process:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'name': INSERT INTO {users} (uid, name, pass, mail, theme, signature, signature_format, created, access, login, status, timezone, language, picture, init, data) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12, :db_insert_placeholder_13, :db_insert_placeholder_14, :db_insert_placeholder_15); Array ( [:db_insert_placeholder_0] => 785 [:db_insert_placeholder_1] => [:db_insert_placeholder_2] => [:db_insert_placeholder_3] => [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => 0 [:db_insert_placeholder_8] => 0 [:db_insert_placeholder_9] => 0 [:db_insert_placeholder_10] => 0 [:db_insert_placeholder_11] => [:db_insert_placeholder_12] => [:db_insert_placeholder_13] => 0 [:db_insert_placeholder_14] => [:db_insert_placeholder_15] => b:0; ) in drupal_write_record() (Zeile 7239 von /www/htdocs/******/*********/includes/common.inc).

It seems like the registration module is trying to write some data into the users table. Any ideas what can cause this?
Also, this error occurs only sometimes, in 80% the registration is successful and doesn't cause this error.

I'd be very glad if any of you could address this issue soon, or point me in a direction to fix this - thanks!

Regards, Marc