OpenVPN server fails to start on Debian Stretch

After a recent upgrade to Debian Stretch, my OpenVPN server stopped working. Checking the relevant log file, /var/log/openvpn.log, I found the following not very helpful error message.

Fri Nov 23 16:46:37 2018 OpenVPN 2.4.0 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] built on Jul 18 2017
Fri Nov 23 16:46:37 2018 library versions: OpenSSL 1.0.2l  25 May 2017, LZO 2.08
Fri Nov 23 16:46:37 2018 daemon() failed or unsupported: Resource temporarily unavailable (errno=11)
Fri Nov 23 16:46:37 2018 Exiting due to fatal error

Fortunately, someone already reported this error to debian and found out that the error is caused by the

LimitNPROC=10

in the /lib/systemd/system/openvpn@.service. The LimintNPROC directive in systemd is used to limit the number of forks a service is allowed to make. Removing this limit, might not be the best idea. So, I would suggest that instead of commenting it out, to find out the minimal value that allows OpenVPN to start and work. After some testing, I found that the minimal value that worked for me is

LimitNPROC=27

After editing the /lib/systemd/system/openvpn@.service, you need to reload the systemd service files and restart the OpenVPN server process.

$ sudo systemctl daemon-reload
$ sudo systemctl status openvpn@server

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