OpenVPN is not working and it's impossible to use NordVPN.
What happened? Frankly speaking I have no idea. My configuration was not changed since last two months. I've been updating packages every few days, nothing more.
I'm running OpenWRT on PC as a router (Hewlett-Packard HP RP5 Retail System Model 5810).
I've tried reainstalling openvpn and libcrypto packages and restore configuration from backup but I'm still getting this error.
Moreover I tried to remove and install openvpn-ssl package and remove openvpn configuration but it gave me the same result.
Is there any safe way to upgrade openwrt to the newest firmware without loosing basic configuration like card1 = lan card2 = wan (bacause openwrt runs on PC not real router so I would not want to broke my router again).
As a general rule, do not use opkg upgrade. Unlike a desktop distribution, OpenWrt is not designed or tested to support that, and it will almost certainly break things. To upgrade, replace the whole OS with the newest version.
Your network settings will be saved when upgrading to a new version. All extra packages will need to be reinstalled, but their config files should be kept. I like to make a directory /etc/config/vpnfiles to save files related to a VPN service (their .ovpn file, certificates, and keys). Everything under /etc/config is preserved during an upgrade.
I've done that once and it ended bad. I mean I was forced to configure my PC from the very beginning because I lost my interface configuration so I could not to connect to my PC from other devices anymore.
If you're running OpenWrt bare metal on x86 hardware, the numbering of the ports may change between kernel versions, and you'd have to change the config accordingly. That is unlikely going from 21.02.0 to 21.02.2 though.
Also your Ethernet ports could temporarily stop working if they require kmods that are not in the release build (though it looks like that won't be a problem here since it is using relatively new Intel chips).