L indicates an I/O link issue.
It could be caused by several things.
When the alarm occurs will it restart immediately or does it take several restart attempts?
Check the link LED on all components in the chain. The 1st unit in the chain that does not have the Link LED on could be at fault.
Once it is determined which unit may be the issue check all connections to that unit are secure (24vdc supply, Link cables).
Check the 24vdc and make sure it is not low. If you have one apply a voltage meter with record feature and watch the 24vdc.
Possibly separate the link cable from other wires to determine if noise is an issue.
The last item and least likely could be the beta motor encoder. If the 5 volts is getting shorted in the encoder/cable it can shut down the drive causing a link alarm. So you may want to look at that cable and ensure it is not damaged or contaminated.