I recently installed a new router and in the process, my network’s IP range changed from 192.168.0.x to 192.168.1.x. Ever since, the server and one of my clients don’t “see” each other any more - the client reports that it is not connected to a server and the server reports “Starting backup failed” when trying to start a backup. Interestingly, it works just fine on my second client, the IP range change hat no noticeable effect there.
The server log does not contain any hints - only “ERROR: Sending broadcast failed!” (although I have allowed port 35623 in the firewall). The only setting I have changed on the affected client is to add “–disable_ipv6 true” to the args.txt file.
Did you check the client firewall (home <-> public setting)? New network => maybe different firewall settings. Disable firewall temporarily to confirm this in general.
@uroni thank you for pointing that out 
it was indeed a firewall rule.
@Igor thank you, I didn’t know this column existed! But in my case, the IP address was not shown there until I disabled the firewall. (the second client’s IP address was also not shown, although I performed an incremental backup of it earlier)