Builds prior to 18.5.462 are known to have issues specifically when using Houdini with VPN.

Try upgrading your Houdini build to a recent Daily Build. Also include to install the license server component if you're using a local license server.
Bug fixes are routinely added and are included in these more up-to-date builds.

It is a good idea to periodically check our change log for bug fixes and features released when it comes to the Licensing category. You can even specify the build numbers you would like to check between.

For Windows users:

Check that your antivirus software or firewall is allowing communication between sesinetd.exe and hserver.exe.
These two programs will need to be whitelisted.

For OSX users:

Sometimes remote network routers will change the machine name.
If your licenses installed locally to the machine and the machine name changes, they will become invalid.
If you would like to use sidefx.com to host your licenses, please refer to our Login-Based Licensing FAQ.

To fix:

1. Launch the License Administrator.
2. Select File->Change License Server.
3. Ensure the license server(s) your machine is set to use is correct machine name associated with your licenses.
4. You may need to rename your computer back to the original name your licenses are assigned to.

For all users:

Sometimes you may have an issue where the machine is set to look only to sidefx.com for licenses but you have licenses installed locally.
Use the License Administrator to look to your machine for licenses.
If you would like to use sidefx.com to host your licenses, please refer to our Login-Based Licensing FAQ.

1. Launch the License Administrator.
2. Select File->Change License Server.
3. Ensure the license server(s) your machine is set to use is correct machine name.

Please refer to this FAQ.