Have you installed firmware from other sources than Bitmain recently?

Or maybe your PC or the device you currently use to change the pool is infected so try to change the pool on other device

If the issue still the same try to flash the miner through SD card flashing.
You can follow this guide below on how to flash the miner through an SD card.

- https://support.bitmain.com/hc/en-us/articles/115000216034-L3-L3-and-L3-Control-Board-Program-Recovery

For the SD card firmware, you can download it from here https://service.bitmain.com/support/download?product=Flashing%20SD%20card%20with%20image

Once successfully flash it again through WebGUI on other devices just download the firmware from Bitmain.
Then test again if it solves your issue.