Hi!
I had the same problem as you. I use Linux Mint 19.1 and I'm using the Flatpack version of OBS. When I tried to install the websocket through .deb, the same error appeared. What I did was the following:
1 - I copied the locale from directory /usr/share/obs/obs-plugins/obs-websocket/ to directory /var/lib/flatpak/app/com.obsproject.Studio/x86_64/stable/0069ec300ce09337a585acfcefe0f2ecdfa0efcd08d920b2751c844fd026296a/files/share/ obs/obs-plugins/
2 - I copied the lib /usr/lib/x86_64-linux-gnu/obs-plugins/obs-websocket.so to the directory /var/lib/flatpak/app/com.obsproject.Studio/x86_64/stable/active/files /lib/ and also to the /var/lib/flatpak/app/com.obsproject.Studio/x86_64/stable/active/files/lib/obs-plugins/ directory
After that, it worked normally! I hope it helped you!