Hi, thanks for wiki info. I suppose a device page would be better to be published. I (mis)used the techdata page for that, shame on me.
On topic: Maybe it is a "feature" of flashing a custom firmware through uboot on that device. I don't know if the outcome would have been different when flashing openwrt instead of ROOter on that device. I am new to that, and don't know if partitions are handled differently in sysupgrade vs uboot.
In the Rooter forum user LOM says, that "The ROOter firmware does not overwrite uboot"
I used uboot because it was not possible to flash ATH79 over the running AR71XX image through sysupgrade.
shame on me, i fooled myself. What happened? Entering "192.168.1.1" in firefox adressbar got autofilled to http://192.168.1.1/cgi-bin/luci. The uboot webserver correctly said "page [cgi-bin/luci] not found". I was not able to go to plain 192.168.1.1 (firefox feature or my illiteracy). Opened page in chrome and the main page of uboot was shown.
Sorry folks for being in chicken mode. Everything went well and I am back on stock firmware.
Also Chrome does that (tries to reconnect to the same full URL link even if you type only the IP).
They also try to cache the page and this can be an issue when upgrading the firmware as they keep the old Luci interface cached and then fail to login or do other stuff with weird errors.
It helps if you close the old tab and use a new tab or close Firefox/Chrome and open it again.