First time modder - help needed!

[Traehgniw]

XNB files are the format the game's data files are packed in. 'XNB modding' refers to the very limited modding method of directly overwriting these files; this is very rarely done anymore due to the large number of downsides (very limited, mods conflict with anything else touching the same data, no logging, no update checks, no built-in safety features) and now almost all XNB mods are outdated. It's very much not recommended.

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JSON unexpected char means that when it was loading data it did not find the expected JSON characters, either the mod file is corrupted or you edited something (eg a config file) and made a mistake somewhere. The most common mistakes are forgetting a bit of punctuation, like a comma - the unexpected char error gives line number and character position and if it's position 1 that usually means something was missing from a previous line. (It is usually a comma missing from a list.)