I dont have a solution, but I think I narrowed it down even further. If you look at your webservers access log files, when trying to import the certificate, you will see the following:
[28/Apr/2020:10:51:50 +0200] "HEAD /vpn.ovpn?embedded=true HTTP/1.1" 200 395 "-" "-"
[28/Apr/2020:10:51:50 +0200] "POST /RPC2 HTTP/1.1" 404 628 "-" "-"
The first line downloads the ovpn file and once you try to import the file, it does the /RPC2 POST call, which is not available (at least on my server) - therefore it gets a 404 error back. Not sure what it is trying to do there, but I believe this is specifically for people using the openvpn-as server.
Any update from an expert would be appreciated.