This is an under configuration of the cURL timeout setting within the Nextcloud installation. Everyone is going to receive the same error when they go into the Nextcloud Apps interface and use one-click to download and enable OnlyOffice Document Server as a prerequisite to installing the app for Nextcloud OnlyOffice Connector. The Nextcloud Hub 21 pages explain that it is easy to first download and enable the OnlyOffice Community Document Server, and then enable the OnlyOffice Connector to be able to create, collaborate and edit Office Documents online from within Nextcloud. In other words, these are now functional integrated apps within Nextcloud.

The FIX! What worked for me was to change curl’s timeout at the time of its invoking from the software installer located in: [path to nextcloud]/lib/private/Installer.php.

So, locate the subdirectory where the file Installer.php resides and change to that directory as your current working directory. Then edit Installer.php using Vim or other text editor.

cd /var/www/<path-to-nextcloud>/nextcloud/lib/private/

sudo vim Installer.php

You will see a line that says something like:

$tempFile = $this->tempManager->getTemporaryFile('.tar.gz');
$timeout = $this->isCLI ? 0 : 120;
$client = $this->clientService->newClient();

Change the 120 (seconds) value to some value that is sufficiently longer, say 240 (seconds) or remove the condition and leave it always as 0 (unlimited, but perhaps that is not the best idea). Now, the Nextcloud Community Document Server App will finish downloading and installing.

See thread at: https://help.nextcloud.com/t/community-document-server-curl-error-28-operation-timed-out-after-120000-millisecond/80589/17

tecnicaso offered this fix. It worked for me. Thanks tecnicaso!

One more thing. When you are installing Nextcloud from the website interface, CLEAR the Checkbox that says Pre-Install some other apps, including Calobora, etc. Somewhere in the installation instructions, it says that we cannot install OnlyOffice Document Server if Calabora is or was installed or has left remnants when uninstalled or disabled.