I have generated the address. The wrong IPv6 ULA was generated at the first start, as can be seen in the picture from the first post. I had not changed anything deliberately. Can I have the PIv6 ULA regenerated by OpenWrt?
I've found the following article, which also refers to an incorrectly created IPv6 ULA prefix.
The article refers to an the OpenWrt 12_network-generate-ula. This script is not to be found at /etc/uci-defaults. If I execute the part that generates the ULA prefix manually, I get an incorrect prefix.