Symptoms
Cannot log into Plesk as Domains > example.com > Web Hosting Access > System user with valid credentials:
You have entered incorrect username or password
Cause
This behavior is by design: the system user of domain is not allowed to log in to Plesk.
Resolution
As workaround, in order to allow access to the particular domain it is possible to create additional user account with an ability to log into Plesk.
with