In that time, he has overseen six managerial changes and the club’s relegation from the Championship last season.
Their first home game of the season is against Northampton Town on Saturday.
What did the letter to the supporter say?
The supporter, whose identity has not been revealed to BBC Sport, was written to by Charlton’s duty safety officer Cliff Eager on 2 August.
It stated: “We have identified that certain comments placed on social media websites by yourself have been not particularly constructive.
“Whilst we recognize that everyone is entitled to their own personal opinion it is not helpful when inflammatory comments are posted on such websites.
“The granting of your season ticket will be ‘conditional’ and subject to you receiving an ‘Agreed Behavioural Contract’, which will request that you refrain from posting derogatory or inflammatory comments regarding the club of people representing the club in the future on any social media websites.”
What was Charlton’s response?
A statement issued by the club confirmed the supporter had been written to about their “continued inappropriate behaviour towards members of staff on Charlton’s official social media accounts and foul and abusive language captured on stadium cameras”.
Charlton’s head of matchday operations Mick Everett and Eager met with the supporter on Thursday.
“During the meeting, the level of abuse, which the individual has since voluntarily removed from their Twitter account, was discussed,” the statement read.
“The club also discussed the video footage, after which the individual apologised on several occasions for his behaviour and assured the club that he would not continue to engage in an abusive manner.”
What do fans think of the move?
Since Duchatelet’s takeover of the Addicks, protest group CARD (Coalition Against Roland Duchatelet) has been formed from an umbrella of Charlton supporters’ groups.
Organiser and lifelong club supporter Alan Davis told BBC Sport he was concerned the letter may be “the thin end of the wedge from the ownership”.
He said: “It might be the start of more to come. We know season ticket sales are down this year and I have not renewed myself for the first time in more than 20 years.
“CARD are not seeking to disrupt matches during August, but we will continue to protest.
“As a group, we developed a slogan that Charlton has become ‘the club that hates its fans’.”