Heya, i am facing the same issue.
-My tun0 interface ip is 10.10.14.23
-no firewall is up at machine or network level
-no port redirection for 8443
-my vpn is up and running (i can ping and access the oopsie machine)
-i have successfully uploaded a reverseshell.php and set up the ip 10.10.14.23 ip, and port 8443
-i am listening to port 8443
nc -lvnp 8443
listening on [any] 8443 …
-and when i execute the shell i got
[~/HTB/Oopsie]
└─$ curl http://10.10.10.28/uploads/reverseshelltest.php
WARNING: Failed to daemonise. This is quite common and not fatal.
Connection refused (111)
I am really trying to see what is causing this , no more hint so far 
PS: btw, for those who get a 404 error the files are deleted every X minutes so you should upload it again