Local network is 192.168.9.0/24. Set up non-transparent web proxy with a few custom blacklist url’s such as ebay, tiktok, facebook, etc. Workstations are Win10 Pro. Turn on web proxy manually at the workstations using the ipFire IP address and port (192.168.9.1 and 800). In general when turn on proxy at workstation things work fine. I can get to most websites but not eBay, tiktok, facebook, etc, but printing to a local network Brother printer returns an unspecified error. The print job still in queue but just says ‘error’. Turn off proxy and prints fine, including anything that was sitting in queue. The Brother printers have static IP’s on the local network (example 192.168.9.90). Brother printer properties shows location as “http://192.168.9.90:80/WebServices/Device” so it would appear to me that Brother printer looks like a website on the local network to the workstation??? I need some ideas on where to go to understand what is happening and maybe suggestions on how to fix it 
A possible hint from my side.
I print in the network (Proxy=on) via socket and pot 9100 an everything does well.
If I tried to administrate the printer via http and I had no access. My solution was to add in the global proxy configuration of the pc (I use Linux) “no proxy for printer xxx”.