Firstly, I solved the curl timeout by modifying the installer.php file ( lib/private/Installer.php) and increasing the timeout value from 120 to something sufficiently high (found this solution in a similar Githib discussion).
This step of changing the timeout in the Installer.php file worked for me.I changed mine from 120 to 360 and I was able to download and install it.
I have a fresh and standard NC 18.03 and I still cannot download/enable Community Doc Server. Not working even with timeout (in Installer.php) of 360 or 720. Is there any fix?
The second problem is that the “proc_open” php function is disabled by my shared webhoster by default since “most popular applications do not use these functions and can be operated without limitations”.
Have you checked in your php configuration or with your webhoster if this php setting is enabled?