There is a bug in our installer where the progress bar jumps back and forth when doing an upgrade. We must be handling the information from dpkg incorrectly somewhere.
Works just fine on fresh install though.
Works well now. I tested on Ubuntu 18.04 and 16.04.
Verified that I could reproduce the issue when upgrading from 4.8.1 to 4.9.0. I then verified that the problem did no longer existed when upgrading from 4.9.0 to build 6042 on Ubuntu 16.04 and Fedora 29.
No errors or warnings in thinlinc-install.log.