Hi,
I have the same issue and it is because of new version of openvpn (3.2.11). According the openvpn change log (https://community.openvpn.net/openvpn/wiki/ChangesInOpenvpn23) is new “Restrict default TLS cipher list”. This causes incompatibility with RouterOS (I’m using 6.36rc12).
The “solution” is to downgrade openvpn to version 3.2.10 on your client.
The solution is implement the ciphers in RouterOS.