I installed 18.1 over my current 17.1 installation of PRTG. After then install and reboot i was asked to verify my licence. The server does not have internet access so I selected offline verification. After generating the activation file I copied the contents back on the licence page. I clicked the button to verify the licence but nothing happened.
I had to roll back to 17.1 and still have not successfully updated my version. My maintenance plan does not expire until 10/2018.