Just started happening to me too. I've had to add IP address 255.255.255.255 to exclusions to stop it.
Malwarebytes
www.malwarebytes.com
-Log Details-
Protection Event Date: 11/20/17
Protection Event Time: 8:00 PM
Log File: 70c2c158-ce2d-11e7-a94d-c8ff28ccbb98.json
Administrator: Yes
-Software Information-
Version: 3.3.1.2183
Components Version: 1.0.236
Update Package Version: 1.0.3304
License: Premium
-System Information-
OS: Windows 10 (Build 15063.726)
CPU: x64
File System: NTFS
User: System
-Blocked Website Details-
Malicious Website: 1
, , Blocked, [-1], [-1],0.0.0
-Website Data-
Domain:
IP Address: 255.255.255.255
Port: [17500]
Type: Outbound
File: C:\Program Files (x86)\Dropbox\Client\Dropbox.exe
(end)