The Seoul regional office of the National Tax Administration is reportedly handling the case and will be looking over tax and accounting documents to acquire evidence of unpaid taxes. The report also states that the National Tax Administration is specifically focusing their investigation on profits from JYJ’s overseas concerts.
For one reason or another, I doubt this is anything to worry about. Most K-pop companies come under investigation relating to taxes at one point or another, and either because it’s a mistake or it’s hard to prove, usually nothing ends up coming of it.
Given everything else the company has to deal with right now, though, this probably isn’t the best timing.