Migrations can't be rolled back

Partly solved it myself.
I deleted the migrations table and ran the first migration again.
This time, the version number DID appear in the table.
Rolling back also works now, despite of the fact that $this->migrations->version(0) returns FALSE.
My eventlog table is being deleted by the down() function in the migration class.

Still wondering what to put in the migration config file.
Initially $config['migration_version'] had value 0.
I changed it to

PHP Code:

$config['migration_version'] = '20170826141700'; 


Should I set it back to 0 before I start the rollback ?
It's pretty confusing, and the CI documentation is not very clear on this subject.