One of Nick Carter‘s sexual assault accusers may need to find a new lawyer as she continues to fight the pop star in court … her legal team is asking a judge to let them out of the case due to a breakdown in communication with their client … TMZ has learned.

According to new legal docs, obtained by TMZ, the lawyers representing Shannon Ruth in her case against the Backstreet Boys singer are asking permission to withdraw as her counsel.

The attorneys say they could “no longer adequately represent [Shannon]” due to a “breakdown in communication on fundamental issues of the case.”

They say they believe Shannon moved from Nevada to Washington … but they do not know exactly where she currently lives in Washington.

As we’ve reported … Shannon sued Nick, claiming she was invited on the singer’s tour bus in 2001 when she was 17. Shannon, who has autism and cerebral palsy, claims she was given a “red-colored drink” which she believes was full of alcohol and juice.

She claims Nick took her to the tour bus bathroom and had her perform oral sex … which she says she did while crying. She says the singer took her to his bed and continued to sexually assault her. Shannon says she was a virgin before the alleged incident … and contracted HPV.

Nick denies all allegations of wrongdoing. He says the incident never went down.

As TMZ first reported, Nick claims Shannon was conspiring with another sexual assault accuser, Melissa Schuman, and her father to spread lies about him in an attempt to extort him for a financial settlement.