First of all check if you end up in this stuck state:
... or if you eventually end up with that:
In the latter case, everything works as expected (you could force changing the LAN ip with "Apply unchecked" and reconnect/relogin manually afterwards).
In the former case, something went wrong with the network reconfiguration after rolling back the uci configuration.
While you see ...
...
ifconfig br-lan on the router should show the new, changed IP address.
Once the countdown is over and you see ...
...
ifconfig br-lan on the router should revert back to the previous, original IP address.
The eventually causes the browser to be able to "ping" http://[old-ip-address]/chi-bin/luci/... again which will then trigger this dialog:
If that last dialog is never appearing, this means that for some reason, the br-lan interface is not reverting back to its old IP address, or if it did, some lower level issue is preventing it from communicating with the outside.
If it is stuck in such a state, I'd further try via serial if things like ifup lan, ifup -a, echo f > /proc/net/nf_conntrack are fixing it. It would also be interesting to observer both logread -f and the output of dmesg while such an apply/roolback session is running to see if there are any carrier events, protocol handlers executed and the like.