"That account could not be located." error after new installation

after the 'successful' installaton of modx-2.2.4 i get following error message: "That account could not be located. Check the username and re-type the password to try again."

the log file shows that there appeared an error while creating the admin user.
so i gave the user a username in mox_users, how bob proposed, but no change.

i also repeated the installation after checking permissions - again without a change.

since i had no difficulties to install modx in different environments, i guess the problem is in server settings. but in moment i've no glue to find the conflict.

for any suggestion to trace the error, i would be very grateful