When the inverter displays the alarm "DC-INTF" that means that a DC fault it being detected. This fault can be either internal or external so the troubleshooting steps below involve determining which one it is and how to go from there.
Information that will help determine whether the fault is internal or external:
How many times a day does the fault occur and at what time(s)?
How long was it after the system was installed before the first fault occurred?
Are the PV string voltages and currents within the specs of the inverter? You can check the silver label on the left side of the inverter for these.
Determine if there is a ground fault on one of the DC strings:
Turn the inverter completely off first - AC and DC both
If a battery is connected, pull out the battery fuses so that the battery is not connected to the DC bus
Remove the PV strings with a technician screwdriver so that they are floating in free air - do this one string at a time
With a multimeter, measure positive to ground and negative to ground for every string
If there is stable voltage (not dropping) on one of the leads to ground then there is a ground fault on that line
If there is no voltage to ground or the voltage drops to zero, then there is no ground fault on that string
Also measure DC voltage to ground for the battery, these cables can be left landed if the fuses are out
If the system is using module-level rapid shutdown then troubleshooting will be trickier.
The best way to see if you have a bad MLRSD device causing the DC fault is to operate the system with just one string connected.
If the system does not throw the alarm, then swap out the strings and run it again.
If neither string on their own triggers the alarm but both strings together do, then the fault is likely internal.
If there is only one string with MLRSD, disconnect all of the MLRSD receivers and then run the inverter with full Voc.
If the system does not trigger the alarm when running without MLRSD, then one of the MLRSD receivers is defective. If the alarm does display when operating without MLRSD, the fault is likely internal provided that you have already measured the string voltages to ground.
If there is second inverter installed next to the inverter having the faults, swap the PV strings, let the system run, and see if the other inverter also throws the alarm.
The system has an internal DC fault if the above steps have been performed and no external ground fault or faulty MLRSD receiver have been found to be the cause.