Guys, I apologize for opening the topic.
After that I spent a few more hours performing tests during the night and ended up realizing my mistake. In fact, instead of adding the nickname in "Developers []" on configured.developer.toml, what should be added is the UUID (which can be easily obtained from the list of players of the world).
For anyone else who may fall into this topic in the future, an example of how it should look:
Code
#Broadcasts messages to all developers about changes applied to server configs
broadcastLogs = true
#A list of UUIDs of players who are developers for this server. The players will
#additionally need to have operator permissions. Developers will have the ability to
#edit server configs on your dedicated server. Only add players you trust.
developers = ["Player UUID HERE", "Player 2 UUID HERE"]
#Developer mode. This enables the ability to edit server configs on dedicated servers.
#You will have to restart your server for developer mode to be updated.
############# READ ME ################################################################
#You should only enable this mode only if you are developing the server. It is #
#recommended that this mode is turned off if your server is public or have players on#
#your server you don't trust, as leaving this mode enabled is considered insecure in #
#those scenarios. #
######################################################################################
enabled = true
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Just reminding you to add only trusted players to the list of developers so that unwanted events don't happen.
I really hope I helped anyone who needed this tip.