The Red LED labeled ERR is blinking/flashing on Modicon M241 series PLC programmed in EcoStruxure Machine Expert or SoMachine.
What does it mean?

Product Line
EcoStruxure Machine Expert, SoMachine v4.1+, 

Modicon M241 PLC (TM241C24R, TM241C24T, TM241C24U, TM241C40R, TM241C40T, TM241C40U, TM241CE24R, TM241CE24T, TM241CE24U, TM241CE40R, TM241CE40T, TM241CE40U, TM241CEC24R, TM241CEC24T, TM241CEC24U)

Environment
M241 physical indicator LED light

There are three types of ERR LED blinking:

Regular flash -

The LED turns on for 500ms and then off for 500ms regularly.

This signifies an INVALID_OS controller state There is not a valid firmware file present in the flash memory. The controller does not execute the application. Refer to the Firmware Upgrade section of the SoMachine HTML Help to restore a correct state.

Single flash -

The LED turns on for 1000ms, then off for a short 200ms and repeats this pattern.

This signifies an EMPTY controller state, no program has been loaded to the PLC in this state (usually after a Firmware Upgrade procedure has been done or when a Reset Origin is performed)

Fast flash -

The LED flashes on and off alternating quickly within 500ms at a regular pace.

This signifies an EMPTY after a system error detected controller state.

This state is the same as the normal EMPTY state. But the application is present (as a BOOT project), and is intentionally not loaded into working memory. A next reboot (power cycle), or a new application download, will restore correct state.

For more information, please refer to the SoMachine HTML Help in the section:

Logic Controllers -> M241 Logic Controller -> Programming Guide -> Controller States and Behaviors -> Controller States Description

Additional troubleshooting tips:

1. Log into the PLC
2. Add a Watch List from dropdown View->Watch
3. Add the System Variable PLC_R to the Watch List 
   The full name of the variable is: "<name of the plc>.Application.PLC_R"
   Typically, the default is: "MyController.Application.PLC_R"
4. Expand the list to reveal the elements of the PLC_R structure by pressing the [+] button on the left of the row.
5. Observe the values of the i_lwSystemFault_1 and i_lwSystemFault variables