What worked to me was to change curl’s timeout at the time of invoking it by software installer in: [path to nextcloud]/lib/private/Installer.php. You will see a line that says something like:

Download the release <block> $tempFile = $this→tempManager→getTemporaryFile('.tar.gz'); $timeout = $this→isCLI ? 0 : 120; $client = $this→clientService→newClient(); </block> Change the 120 (seconds) value and put something bigger or remove the condition and leave it always in 0 (unlimited, but perhaps not the best idea). See thread at: https://help.nextcloud.com/t/community-document-server-curl-error-28-operation-timed-out-after-120000-millisecond/80589/17 tecnicaso's offered this fix. It worked for me.