Whatever Mueller is going after Manafort about is stuff he did 10, 15 years ago. Shady stuff, illegal financial stuff with Ukranian oligarchs or whatever.
So what.
He’s going after him because he wants to deny Trump the ability to hire anyone, ever. Very few in DC are clean enough to escape Mueller’s scrutiny. Even those that are, like Flynn, Page and Papa G are prosecuted. If you tell him the truth, he gets someone else to lie and then says you are the one lying, like he is doing with Manafort now.
It’s about more than a little failure to file, which the DOJ already looked at and decided was unimportant 7 years ago. It’s part of a plan to destroy Trump, by destroying those around him. It needs to be stopped.