Hello, tl;dr: How to change your Disable, network device, Physical Address to an Enable, network device Physical Address on the Unity License so that the license activate properly and you can open legacy editor projects, if you get title's error. If by any chance you are having problems with opening some projects from older versions of Unity, for example(in my case): Can Open 2019.4.31f1 LTS projects But cannot open projects from 2019.2.21f1 Because: Every time you click on the project on the Unity Hub, the launcher disappears and the Unity logo appears and starts to initialize, just so seconds later the Unity Hub flashes and Unity doesn't load or looks like it crashes. You can CREATE a new project using the legacy versions of Unity but if you close it, and try to open it again, the same problem happens with a CLEAN project. If when you Create a new project on the legacy version of Unity and go to the Help menu and select, Manage License and Unity shut down and/or look like it crash instead of opening the License Management section on the Unity Hub. Then you might be getting this error: If this or something equally close might be the case this post: https://forum.unity.com/threads/stu...re-first-unity-is-already-in-list-etc.903851/ Might help you to track it down and solve it. However, if by any chance NONE of the errors listed on that POST, show on the: Licensing logs: %LOCALAPPDATA%\Unity\Unity.Licensing.Client.log Editor logs: %LOCALAPPDATA%\Unity\Editor\Editor.log Hub logs: %APPDATA%\UnityHub\logs\info-log.json Meaning that all your logs show that your Licensing Activation Program is running successfully and nothing shows on the logs as a possible error with the license. Then Unity Hub, when doing the Online Activation method, MIGHT be feeding the Physical Address of a DISABLE Network Device on Binding Key="5" on the .ulf licensing file of Unity. Example Physical Address data on file: <Binding Key="5" Value="a1:2b:33:4d:05:6e"> I think it is because the License Activation Program send the physical address for activation of the first network device on the PCI bus, in my case I have two Network Devices integrated on my MOBO, and one is completely disable, because I don't use it. In this case my primary network device was, a WIFI integrated network device that was disable because I use my Ethernet integrated network interface. This cause the issue of feeding the wrong Physical Address here: Now, MY SOLUTION TO THIS PROBLEM: Manual Activation on the Unity Hub Follow the process and this will provide you with a Unity_lic.alf file. Open it on a text editor and look for the Binding Key="5" section that will have Value follow by the current Physical Address provided. Example: Now, look for the Physical Address of your Network Device that is ENABLE and you are currently using to navigate the internet. (Either ipconfig /all or go to Network Connections on control panel): Be sure to check that the Physical Address that you are tracking on your network details IS from the Network Device that is enable and then change the Physical Address on the Unity_lic.alf text file to the one Network device that is Enable. Then save the file (be sure you wrote that address properly after Value inside " "). Then go to the Manual Activation website of Unity: https://license.unity3d.com/manual And slam in the Unity_lic.alf text file. After the process is done you will receive a new Unity_lic.ulf text file, open it, and be sure that the Physical Address of the Enable network device is properly set on Binding Key="5", if not try again, check if you are saving the changes before upload. Then go back to the Unity Hub Manual Activation, and add the new Unity_lic.ulf and you should get activation. After that, you should be able to open projects from legacy Unity Editor on the Unity Hub. On a side note I contacted and workout this problem with Unity Support, so they know about this issue, specially the part where NONE of the logs show problems with the licensing and that the error message appears on the Unity Hub for a millisecond so you need to time a perfect screenshot to get evidence on the error message on Unity Hub. I hope this post can serve anyone, with the same or identical issue.