Ubuntu Server 18.04.2 LTS, Tools install but do not activate

Working with Parallels 14.1.3, on Mojave. Client OS is Ubuntu Server 18.04.2 (18.04.1 didn't work either). I can install the Parallel tools successfully. After reboot, the tools do not seem to be active: The mouse is captured permanently, and copy/paste does not work.
The only hint of a failure is this message during boot:
prl_tg: module verification failed: signature and/or required key missing - tainting
Does anyone know what needs to be done?