Shared discs

Hello guys,
I have a strange issue. Iam using mikrotik RB750 OS 5.11

  • Windows Server 2008 (static IP)
  • Clients (mixed XP and Win7 - DHCP IP from Mikrotik)

Internet connection is working great, but sometimes there is a problem with WinServer shared disks. When client release IP from mikrotik, shared discs are not visible, till the IP is changed at client to static IP. Than I can set it back, to release IP from Mikrotik.

This is really strange because I have no idea where can be the problem. (Problem most occurs with WinXP).

Thanks for any idea.