After upgrading my OpenVPN server to CentOS 7.5 I had trouble connecting to it.
Specifically, I had two different issues:
** the laptop, which is running Fedora 28, was able to connect just fine but DNS resolution was broken.
** OpenVPN for Android was also connecting just fine but reporting a weird error: OpenVPN: tun tap invalid argument (code=22).
The first one was caused by me because after the CentOS upgrade procedure was completed I also run yum autoremove which deleted dnsmasq; the solution was fairly simple, reinstall and reconfigure dnsmasq.
For the second issue the solution was to enable comp-lzo and voilà, everything started working again.