Hello there,
As I read in instructions, in mikrotik user-manager login page, the username is “admin” and the password is blank.
But I have"invalid username or password" error!
I set the user-manager owner password (/tool user-manager customer add login=aaa password=aaa permissions=owner) but nothing changed. 
hello
do those from user manager in customers
then use the command in terminal
too user manager customer pr
to show the password you can use this
/tool user-manager customer print;
if you want to change the password for admin:
/tool user-manager customer set 0 password=aaa;
good luck.