This is expected behaviour .
Because the . and .. are treated as special parameters . i.e . -> points to current directory and .. points to previous directory .
So when you say http://www.question2answer.org/qa/user/. it is exactly same as saying http://www.question2answer.org/qa/user/ .
And also when you say like http://www.question2answer.org/qa/user/.. same as http://www.question2answer.org/qa/
So this is causing the behavior . I am not sure what is the use case of this as no body wants dot or double dots to be their username .
If you want to block the user names you can use this plugin - https://github.com/amiyasahu/q2a-registration-blocker
If you want to read more about the URLs - http://stackoverflow.com/questions/9367686/dots-in-url-or-path
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April 20, 2021: Q2A 1.8.6 released!