Post as much detailed information as you can about the connection between the PV+ terminal and the controller.
What firmware revision is the PanelView Plus ? What firmware revision is the 1756-L55 ? Has the system been reviewed for backplane chip problems (the notorious Atmel bridge chip issues) ?
Has this problem always existed or has it recently developed ?
It's unusual to get a LegacyDP error with ControlLogix because usually a PanelView Plus uses Ethernet to talk to a ControlLogix using native CIP protocol, not the "Legacy" PLC-5 protocols.
But you might have some kind of PLC-5 address emulation going, or a DH+ link, or something else unusual.
LegacyDP communication errors were common ten years ago because DH+ links were more common, and because early PV+ communication drivers were less stable than they are today.
If this were my system I would be looking closely at the communication link to find out if I had bad wiring, or find a way to decrease the load on the network.
You can keep the problem from vexing your operator by preventing the communication error from popping up the [Diagnostics] display.
In FactoryTalk View Studio, the Diagnostics List Setup allows you to configure when the Diagnostics display will be opened up automatically. The defaults are when an Error or a Warning occurs. If you un-checked Error, the Diagnostics list shouldn't pop up automatically.
This won't fix the problem, but will make it less visible. If you really do have physical media problems or link capacity problems, you'll still have to deal with them eventually.
(aside: 10,000 posts ! I have DeviceNet to thank.)