Blank page on AOSP browser Android 6.0

I'm not sure I understand your message. Can you clarify what you mean when you say you get a blank page from an API request? Dropbox API requests involve communicating with the Dropbox API servers, typically exchanging JSON, not HTML web pages, over HTTPS.

Are you referring to the Dropbox app authorization page itself (at https://www.dropbox.com/oauth2/authorize...)? That's not an API request, but rather just part of the Dropbox web site, used during the OAuth app authorization process.

If that's what you're referring to, please note that that web page is subject to the overall dropbox.com system requirements. That doesn't officially support the "AOSP browser", so I recommend upgrading to a newer, supported browser.

The problem is that with the default browser pre-installed in Android 6.0 stock the dropbox.com website (and so also the authorization app page) show a blank page, so i can't authorize my app.
With the default browser on Android 4.4 stock all works good. 
One month ago also with browser pre-installed in Android 6.0 stock all was ok...

I see, thanks for clarifying. 

Unfortunately, as that browser is not officially supported, I can't promise a fix, but I will bring this up with the team.

I just tried this myself with an Android 6.0 emulator, and I'm not seeing the behavior you're seeing:

Can you share the exact User-Agent for the browser you're using, as well as any JavaScript errors from the page?

Here, more info:

Browser versionAndroid versionConsole error

There is a workaround,

If i flag "Request Desktop Site", the page works good

UPDATE: 

...but the Sign-In process doesn't work. 

Thanks for the information! Can you share the User-Agent string though? You can get it by going here in the browser, for instance:

https://www.google.com/search?q=What%27s+my+User+Agent

Real device:
Mozilla/5.0 (Linux; Android 6.0.1; Max2 Build/2018-12-26_19-06_2.0.1_0d1b2fc; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/44.0.2403.119 Safari/537.36

Emulator:

Mozilla/5.0 (Linux; Android 6.0; Android SDK built for x86 Build/MASTER; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/44.0.2403.119 Safari/537.36

Thanks! That's helpful. I'm bringing this up with the team. Again though, I unfortunately can't promise a fix as this browser isn't officially supported. 

Thanks!