After DTS modification, the router boots.
For future reference: the dts openwrt/target/linux/ath79/dts/ar9341_tplink_tl-wr841-v8.dts was modified following way:
starting on line 52:
partition@20000 {
compatible = "tplink,firmware";
label = "firmware";
reg = <0x020000 0xfd0000>;
};
art: partition@3f0000 {
label = "art";
reg = <0xff0000 0x010000>;
read-only;
The size of the firmware partition was modified to 0xfd0000 and the beginning of the art partition was modified to 0xff0000 ( 0xfd0000+0x020000).
After this modification the sysupgrade image was uploaded and flashed to the device.
This post provides explanation why the 18.06 method of compilation was not working anymore.