I know this is a pretty old thread, but I just faced this issue in my lab and here's how I was able to get over it. Posting it here just in case I forget what I did or if someone else stumbles on the same. 
1. Reset the root password using the procedure mentioned in the article below. The important thing to note is that you have to make sure the GNU GRUB menu looks exactly like it is shown in the menu screenshot embedded in the article. Without this, I kept getting the hv_balloon... floor reached messages and the root prompt never showed up. This was the second time that I had to do the root password reset, but the first time I did not hit the hv_balloon messages issue. So, keep this handy in case you do.
https://docs.vmware.com/en/Unified-Access-Gateway/2203/uag-deploy-config/GUID-F1B90F5A-DC1A-4175-B34...
2. Next step is simpler actually. Reset the VM and let it come back to the blue VMware EUC Unified Access Gateway screen. Then, just hit enter and login with root. The adminpwd command should work from here. Even I could not fully understand what changes that allows the command to work from here, but it does work! That's how I reset my admin password.