That suggests that something else is using that address and port combination.
Try running netstat and see if port 8953 shows up in the output and if so what is shown as using it.
On my system I get
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 ipfire.domain:54570 dns2.digitalcourage:853 ESTABLISHED
As you say it should be using port 853.
You don’t have any firewall rules forcing the use of your DNS servers do you.
If yes, then maybe check the port numbers that have been used for a typo.
If no, then we are getting close to the limit of my knowledge.
Will wait to see what netstat shows.
You can ignore what I put above.
Netstat won’t show any DNS server connection because unbound can’t start. Also we are not interested in what is using port 8953 because the DNS servers should be using 853 as you mentioned.
The only unbound file I find that mentions the port is
This stores the servers you have listed in the DNS WUI table.
Mine looks like this
# This file is automatically generated and any changes
# will be overwritten. DO NOT EDIT!
You can see that port 853 is specified for each of the servers.
If one of yours has 8953 in place of 853 I would suggest deleting that server in the WUI table and recreating it. If they are all 853 then I don’t have any further ideas.
I did a quick check of the unbound.conf file and it does not mention the port number.