virtualbox.org

I have vbox 4.3.14 on Windows 8.1 Pro x64.

The menu Help / Check for updates... claims I have the latest version. But there are already two newer releases available (4.3.16 and 4.3.18).

Is there some issue with the update server?

I don't have other network problems.

I have 4.3.12 and have the same symptom.

I've not seen anything official that says versions after 4.3.12 are not offered by the upgrade server, but I expect these are purposely not offered due to the security hardening of 4.3.14 causing many headaches for users.

Since you have 4.3.14 already, you should be safe to upgrade manually to a later version via website download, but backup and test before rolling out to production

Upgrade offers will return as the bugs are worked out I am sure.