TL:DR: I sucessfully removed the error 9003.

I have been trying to solve this issues for many months and I'm so happy that I found this thread. When I contacted TP-Link support they were completely unhelpful and had no idea what this error code was. They suggested I return/replace my device.

I have an Archer VR400 router and I can confirm that the error 9003 is related to having a Broadlink RM Mini3 device that is connected to the network. I don't know why, but removing this device has cleared the error.

To remove the 9003 error code:

1. Disconnect the Broadlink RM Mini3 from the power (shut it off)
2. Reboot the router
3. Log back into the router settings - no more error.

Link to another thread with multiple users indicating that the Broadlink RM Mini3 was causing issues.

Thanks to @epyblast for suggesting this solution!