Upgrade to Trisquel Mini 8 failing

,,, with this description:
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Trisquel
* Running the current pre-release version of Trisquel
* Unofficial software packages not provided by Trisquel

but it is not telling me which applies, so I do not know which to attempt to remedy.

Should I uninstall Dropbox, Megasync, and overGrive, my cloud connections? Calibre? And reinstall them afterwards? This is frustrating, as my old EeePC does not now want to even try to boot from the USB stick, so I can't even attempt a clean install.