I had exactly the same issue after updating to 5.3.0 and rebooting the device. Unmounting in LuCI didn’t work, reformatting the mSATA drive in reForis didn’t change anything, several reboots of the Omnia didn’t change this.
I solved this by removing the mount point in the “Mount Points” section in LuCI (go to the “Mount Points” page, scroll to “Mount Points”, delete the mount point (e.g., /dev/sda/)). I then rebooted the device, and could access /srv again. Interestingly, formatting the mSATA drive in reForis didn’t work (the web UI said it was formatted, but all the files were still there).