@Firestorm I’m not sure if this helps but in section 8.1.2 of the administration manual says the following about “Server URL” :
"URL to which the client will browse if a user selects “Access/restore backups”. For example “http://backups.company.com:55414/”. Default: “” (If empty “Access/restore backups” will not be available on the clients.) "
I believe this is what is producing your error.
It seems that you just need to type the URL you use to access the UrBackup web interface in there to fix this
If you access UrBackup by its IP then put the IP there, if you use some sort of hostname.domainname url then use that.