Orbis: https://minecraft.curseforge.com/projects/orbis
./gamemode 4 command instead of switching me to orbis mode just switches me to survival mode.
This issue appeared only after installing FE on the server.

Use /toggledesigner instead

Forge Essentials overwrites the game-mode command to provide additional functionality, so if Obris does so as well that is probably causing a problem. We can look into providing Orbis compatibility at a later date.

This is a bug in Orbis's code.
https://git.gildedgames.com/GildedGames/Orbis/blob/master/src/main/java/com/gildedgames/orbis/common/OrbisDeveloperModeEvents.java#L32

Orbis is only doing it's gamemode 4 logic if the class is CommandGameMode. Since Forge Essentials overwrites the Gamemode command, this never gets called.

Orbis should update their code so it supports overwriting the gamemode command. I would recommend them replacing line 32 with
if ("gamemode".equals(event.getCommand().getName()))
to support overwriting the gamemode command.