Hmm - you must get the required update to FTM installed first. When you say "log on" to FTM, are you simply opening the program, with no trees open and not logged in to ancestry?
It is perfectly possible to close FTM (Exit) without logging off from ancestry or closing the tree you've been working on. This is often convenient as FTM simply re-opens the last tree and reconnects to ancestry when next opened up - except when you need to get things like the updates installed.
Try closing down FTM by closing (File Close) the tree and logging off before exiting. When you next open FTM don't open your tree, just login to ancestry. That should(!) bring up the "upgrade required", and enable that to take place without any attempt to sync trees, which may be confusing things.
If that fails, make sure you have your tree files, plus associated media folders and backups all safe, then do an uninstall, reboot and re-install FTM. Then login/connect to ancestry, and it should get the update, before you open any trees. Once that's settled down, open a tree file (via File Open rather than importing a 'new tree'), and FTM should then continue and sync. Why the same prog never works exactly the same on any two machines is another matter!