'Unknown Error' can appear if the network connection to the printer is not stable. The printer dialog opens, but then shows 'Unknown error' and closes again.
You can test the above by temporarily changing your printer's IP to localhost in Settings -> ServerAdmin -> select your printer -> Configure.
Try to open the printer dialog again. The printer dialog should now open up without giving an 'Unknown error'.
Solution:
- Fix the network connection (for example: reset/replace switch, network cable, enter a valid IP etc).
- In some rare cases you might also have to reset the printer driver. Some printers models can also be reset as shown here.
You might also get Broken Pipe errors when getting Unknown Error.