I've the same error, that's how I fixed it:
1. search the text "Could not create a valid and unique handle from" in q2a codes, you'll find it in qa-include/qa-app-users-edit.php
2. type echo before row $error=$filtermodule->filter_handle($handle, null);
echo $error=$filtermodule->filter_handle($handle, null);
3. Refresh page, I've got error "maximum length is 20 characters". This means that I put big users handle name in function qa_log_in_external_user()
4. Solution is substr( handle, 0, 20) or try to make handle bigger:
mysql> ALTER TABLE `qa_users` CHANGE `handle` `handle` VARCHAR( 30 ) NOT NULL;
in the file qa-db-maxima.php change constant QA_DB_MAX_HANDLE_LENGTH to 30 (for example)
