18F4550 usb suddenly stop communicating

[broque]

I control Siemens micromaster 420 from my circuit via rs485 and get motor parameters from computer on usb.

I use both usb and uart communication on pic18f4550.. Created a software from Delphi using winusb.dll
OS: Windows 7 32bit

In the beginning everything is perfect. USB enumerating, my Delphi program successfully communicate with pic, pic successfully communicate with micromaster. But sometime pic suddenly stop communication on usb. USB don't detach and Delphi program still sends data to usb. If this happens I simply take out usb cable from computer and attach again.. After that communication continues successfully.

What is happening and usb stops communication suddenly?

I tried to disable rs485 and everything is fine. When i enable rs485 communication, the error above encounters.
Maybe it is about back flows or something like that!

Thanks for any help!