When I try to log in, I receive an error message “Your username or password is incorrect”. What should I verify?

...I verify that I haven't entered the wrong username or password. 
Be careful with upper and lower case! If the key  caps lock shift on my keyboard is activated, the username or password entered will almost certainly be wrong - I can click on the button to the right of the password to view it and verify that I did not make a mistake. 

...I verify that I'm connected to the right Parents Webspace.
If I have several children enrolled in different institutions, the address to which I connect and my login credentials are different!

...I verify the connection mail sent by my child's institution.
Some institutions ask their users to change their password every year: in this case, my password from last year is no longer valid. My new temporary password was sent to me at the start of the school year by post (or possibly by e-mail or SMS).