XCTU can't connect to the internet; timeout when fetching updates

Whenever XCTU tries to access the internet (check for updates on program start / updating firmware library), a timeout error occurs.

This is a problem, as I need to configure a new XBee3 LTE-M Module and XCTU requires to update the firmware library before showing the configuration of the module.

The problem occurs even with Windows Firewall disabled and running XCTU as administrator.

Do you have any ideas what could cause the issue, or how to workaround? Any help appreciated.

XCTU
Version: 6.4.0
Build ID: 20180615-5

Operating System
Windows 8.1 64bit