while digging deeper I've got the solution --
if anybody else stumbles into this problem sometime:
Unfortunately the parameter file_uploads in our php.ini file was set to off.
Even if the avatar pics are stored in the sql database the php file-upload must be set to on to enable the super-global variable $_FILE which is used to handle the file-select process for the avatar picture (in qa-page-account.php)
file_uploads = On
and of course the directory defined via "upload_tmp_dir" must exist :)