After entering the code provided by Dropbox, Joplin says:
Could not authorise application:
request to httрs://api.dropboxapi.com/oauth2/token failed, reason: connect EACCES 162.125.70.19:443
Please try again.
Joplin version 2.3.3 (prod, win32) at Win10 LTSB v1607.
Joplin for Android works just perfect.
UPD: Problem solved. It was just a firewall error.