I had this error when updating to the latest stable version but the beta has this changelog
2. Fixed MT1300 cannot relay Fritzbox's wifi.
so this one is definitely fixed
I don’t see the router rebooting, the process goes as you described…I counted the boot time, and it takes around 1 and a half minutes so I think it is fine, I was just overreacting since I swear in the previous version (3.203) was faster than that but this is definitely not a long time