I set my WD MyCloud Mirror to NOT update firmware automatically. Therefore, I just upgraded last Friday (9/6/2019) to 2.31.183.
I had to deal with the SAME ISSUE!
The fans on the device running constantly was the first indication. When I connected to the GUI via the local web address, it would hang, timeout, not let me get into sub-menus.
Well, after lots of messing around with it, I was able to finally DISABLE Cloud Access, by quickly navigating to that menu right after a reboot. I have always had that feature disabled (based on other posts I believe it was enabled when I recently used the mobile app to transfer some files).
That helped a lot, but it was still not working properly, the GUI was not responsive, it just seemed like something was taking all the real-time or there was an I/O bottleneck.
Finally, I disabled the Media Server and everything started working perfectly!
So, I believe the core issue I was dealing with, and possibly you as well, is something regarding the operation of the Media Server being changed (broken) with this new Firmware.
Right now I’m okay as I use the device primarily for MacOS backups and media file storage (basically as a NAS).
However, VERY disappointed that the Media Service is now unavailable!!!