Page not found when click on UM “User” page

Today I discovered a problem on my site that when I clicked on the UM “User” page, it shows Page not found error. The User page content is [ultimatemember form_id=???] only.
The link URL is:
https://www.paragliding.org.hk/en/user/
When user click is link, it will becomes: https://www.paragliding.org.hk/en/user/%5Busername%5D
After that the page show Page not found error. It was working before but I don’t know why it has problem now.
Could you please help me to resolve this problem? Thanks!