Thanks to @ccxCZ on #[email protected], I could find what was the problem.

Using the following:

tmux show-options -g | grep prefix

Showed me that the prefix was:

prefix `

Testing it, it was indeed this. I don't know how it turned out this way.

Since my tmux.conf was right, I just had to do:

tmux source /etc/tmux.conf

And the prefix was set back the right way.

Thanks to @ccxCZ on #[email protected], I could find what was the problem.

Using the following:

tmux show-options -g | grep prefix

Showed me that the prefix was:

prefix `

Testing it, it was indeed this. I don't know how it turned out this way.

Since my tmux.conf was right, I just had to do:

tmux source /etc/tmux.conf

And the prefix was set back to the right way.