There are many reasons why you would get a license error. Here are some of the most common.
Your license has expired
Solo licenses do not expire. Contact sales@HoudiniESQ.com (919.926.0755) to purchase a new license
Your computer's date isn't correctly set
The computer hosting HoudiniESQ must reflect the correct time and any client connecting to it must also.
The HoudiniESQ Service does not have internet access
As per the EULA electronically signed HoudiniESQ must have internet access to run
You attempted to add more seats than your license allows
Contact support@HoudiniESQ.com (919.926.0755) to request re-registration of the product
The Houdini server is having a permissions error
This occurs on OSX. You must shutdown the HoudiniESQ server and run Repair Permissions then restart the HoudiniESQ service see how to restart HoudiniESQ