When logging in, just input text, it will be fail.

Unfortunately, "XBrowser" is not an officially supported browser for the Dropbox web site. You can find the list of supported browsers and versions for dropbox.com here. I'm afraid we can't offer support for any browsers/versions not on that list.

I created a new App and used WebView, but when I log in to Dropbox(https://www.dropbox.com/login), an error occurs when I type xx@x.xx (x means any English word), such as video.(https://drive.google.com/file/d/11bIzdt7LD5vk1-2X0AG7vX6yAAnb8SdN/view?usp=sharing)

Why is Chrome browser without problem, but my App use WebView can't?
(I remember that it was still available 3 months ago, but the error only started to occur recently)

Web views often don't use the same browser engine as the system browser installed on the system, and so get out of date and are prone to compatibility issues like this. The Dropbox OAuth app authorization flow is accordingly not supported in web views like this. You should only process it in a supported system browser. You can find information about this in the /oauth2/authorize documentation.