The BBC reports that the issues are related to financial fraud:

Lou Pearlman, the man behind boy bands ‘N Sync and the Backstreet Boys, is set to plead guilty to a $300m (£152m) fraud scheme, prosecutors have said.

[…]

The Backstreet Boys and ‘N Sync eventually sued him, claiming he was siphoning large amounts of money from them. The cases were later settled, but the terms were not disclosed.

Frankly I think a case could be made for lyric and tone fraud as well, but maybe it was not entirely Pearlman’s fault:

Keepin it inside, it’s killing me
Cause all I ever wanted comes right down to you (to you)
I wish that I could find the words to say
Baby I would tell you, every time you leave
I’m inconsolable

Um, brokenhearted?