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"
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 ??