Currently with the 1.18.8 update and G-portal servers the same problem.
"Server actively refused" error.
No whitelisting, tried player auth toggle, didn't work, wiped the server and made a new one, didn't work.
Server says it's on 1.18.8 version. But no deal.
It's probably an issue on G-Portal side, because my other server works, and single player works.
Also, vanilla setup, no mods.
I tried any form of setting and nothing works.