While an employee tries to book for a course via ESS, he gets error 'Enter a valid participant.'
The employee is active in the system. Hence not sure why this error is coming.
There are a few other things you can try:
Go to the IMG > Personel Management > Org Mngt > Basic Settings > Data Model Enhancement > Relationship Maintenance > Specify Start Date as Variable for Person Data.
Are the checkboxes for 'Fr.birth' and 'Till highd' checked?
There are also notes 1821759, 1831209.