I just started using the HMI.  I added an extra DigitalPanelMeter #2, reading the variable N7:0. 
When I run the program it shows the value for N7:0 in the beginning, but once in a while I get the message
"INVALID VALUE RETURNED,  Requested source in use".  Then suddenly the program stop running and I get the messageIO Exceptiontion was unhandle" &
"The requested resource is in use".

Using: AdvancedHMIBetav326
           Micrologix 1200 with two serial ports
           DF1 Full Duplex, on Channel0

DigitalPanelMeter1 is reading T4:0.acc
DigitalPanelMeter2 reading T4:1.acc no problem
DigitalPanelMeter2 reading N7:0, or F8:0 I get the error.

Removed DigitalPanelMeter1, and reading F7:0 with DigitalPanelMeter2, then it works OK.

I tried to stop (unload) RSLINX, still the error.

Any ideas what to try next?