Is there any "block_user" module? After moving my sites to the new server, and some updates in the mean time. I have found one of the sites displaying the 

"The following module is missing from the file system: block_user bootstrap.inc:276"


warring.

Have search the DB and it seems the only "block_user" string is located inside the key_value table, it's raw definition looks like that:

INSERT INTO `key_value` (`collection`, `name`) VALUES ('system.schema','block_user')


It has one more binary column as well.

To test the issues have changed the field value to "block_user_1234", it changed the warring message so it's the point for sure.

Should I remove the row ??
There is no block_user module so far as I can see or find. 

You can delete the row from the key value table, yes. It should get rid of the error. You can do it with drush if you want.


Back up your database
drush sql-query "DELETE FROM key_value WHERE collection='system.schema' AND name='block_user';"


This assumes a Drupal 8 site.