What is the ruling on repeated conversations between a man and a woman via chat regarding matters of religion and advice, with no seclusion involved and limited to brotherhood/sisterhood in Allah?

Speech between a man and a non-mahram woman without necessity is a gateway to fitnah (temptation/strife), even if it is about religious matters, as the scholar Al-Khadimi said: "Speaking with a young non-mahram woman is not permissible without necessity because it is a place of suspicion for fitnah."

Therefore, it is not permissible for you to converse with this man except for work necessity, while observing the limits of Islamic law, with the reminder that the default for a woman's work is not to mix with men.