Issues after a migration of sites

3 days ago I moved two joomla sites (one J2.5 and one J3.4) from their previous server (cPanel) to a Virtualmin VPS. All DNS & VPS settings are correct, and for anyone that hasn’t visited these websites before, they display nicely.
But, visitors that had previously browsed these websites, see only the bare HTML output, without CSS or images loading. All website files are in place, nothing seems to be missing or corrupt.
Also, on the same computer (Mac) with 4 browsers installed, it looks great on Safari and Opera (haven’t used these before) but on Firefox/Chrome (regular browsers) the problem persists.

Have flushed all local DNS cache, browser history etc but no improvement. PHP version on both servers is the same, have also flushed Joomla cache.

Any ideas?