WRT1900AC / 19.07.2 can't save settings

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My Linksys WRT1900AC can't save settings. It is running 19.07.02.

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 2560      2560         0 100% /rom
tmpfs                   126024       296    125728   0% /tmp
/dev/ubi0_1              27064        48     25600   0% /overlay
overlayfs:/overlay       27064        48     25600   0% /
ubi1:syscfg              31536       232     29660   1% /tmp/syscfg
tmpfs                      512         0       512   0% /dev
root@OpenWrt:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00100000 00020000 "u-boot"
mtd1: 00040000 00020000 "u_env"
mtd2: 00040000 00020000 "s_env"
mtd3: 00100000 00020000 "devinfo"
mtd4: 02800000 00020000 "kernel1"
mtd5: 02500000 00020000 "ubi"
mtd6: 02800000 00020000 "kernel2"
mtd7: 02500000 00020000 "rootfs2"
mtd8: 02600000 00020000 "syscfg"
mtd9: 00780000 00020000 "unused_area"
mtd10: 00008000 00008000 "spi0.0"
root@OpenWrt:~# 

[pleappleappleap]

I finally solved it. I had to wipe the entire content of the flash by dd'ing zeroes, then I installed the Linksys stock firmware. I rebooted into the stock firmware and used it to install OpenWrt 18.06.8. Then I used the upgrade image to bring it back up to 19.07.2. Now everything works and the performance has improved.