If it happens when trying to remove de ip address e.g
ip addr del 2a01:xxxx:xxxx:xxxx::2 dev enp35s0
try specifying the subnet mask like so
ip addr del 2a01:xxxx:xxxx:xxxx::2/64 dev enp35s0
dmesg log:
~$ sudo dmesg -H -w
...
[pro15 19:12] RTL8226B_RTL8221B 2.5Gbps PHY r8169-0-2a00:00: Downshift occurred from negotiated speed 100Mbps to actual speed 10Mbps, check cabling!
[ +0,000012] r8169 0000:2a:00.0 enp42s0: Link is Up - 10Mbps/Full (downshifted) - flow control off
[ +2,048824] r8169 0000:2a:00.0 enp42s0: Link is Down
[ +8,959020] RTL8226B_RTL8221B 2.5Gbps PHY r8169-0-2a00:00: Downshift occurred from negotiated speed 100Mbps to actual speed 10Mbps, check cabling!
[ +0,000014] r8169 0000:2a:00.0 enp42s0: Link is Up - 10Mbps/Full (downshifted) - flow control off
[ +0,163394] r8169 0000:2a:00.0 enp42s0: Link is Down
[ +1,240247] audit: type=1107 audit(1702663934.240:136): pid=1078 uid=103 auid=4294967295 ses=4294967295 subj=unconfined msg='apparmor="DENIED" operation="dbus_method_call" bus="system" path="/org/freedesktop/timedate1" interface="org.freedesktop.DBus.Properties" member="GetAll" mask="send" name=":1.113" pid=2106 label="snap.firefox.firefox" peer_pid=5540 peer_label="unconfined"
exe="/usr/bin/dbus-daemon" sauid=103 hostname=? addr=? terminal=?'
[ +7,588296] RTL8226B_RTL8221B 2.5Gbps PHY r8169-0-2a00:00: Downshift occurred from negotiated speed 100Mbps to actual speed 10Mbps, check cabling!
[ +0,000013] r8169 0000:2a:00.0 enp42s0: Link is Up - 10Mbps/Full (downshifted) - flow control off
[ +1,259109] r8169 0000:2a:00.0 enp42s0: Link is Down
[ +8,866622] RTL8226B_RTL8221B 2.5Gbps PHY r8169-0-2a00:00: Downshift occurred from negotiated speed 100Mbps to actual speed 10Mbps, check cabling!
[ +0,000013] r8169 0000:2a:00.0 enp42s0: Link is Up - 10Mbps/Full (downshifted) - flow control off
[ +0,047328] r8169 0000:2a:00.0 enp42s0: Link is Down
[ +8,784982] RTL8226B_RTL8221B 2.5Gbps PHY r8169-0-2a00:00: Downshift occurred from negotiated speed 100Mbps to actual speed 10Mbps, check cabling!
[ +0,000012] r8169 0000:2a:00.0 enp42s0: Link is Up - 10Mbps/Full (downshifted) - flow control off
[ +0,214446] r8169 0000:2a:00.0 enp42s0: Link is Down
[ +11,364429] r8169 0000:04:00.0 enp4s0: Link is Up - 100Mbps/Full - flow control off
[ +0,000016] IPv6: ADDRCONF(NETDEV_CHANGE): enp4s0: link becomes ready
[ +0,500115] r8169 0000:04:00.0 enp4s0: Link is Down
...
Check the cabling. Plug/unplug the cables from the router and other devices. After this the problem was resolved.