That's strange.
Are you using a different login account for GNOME Classic or the same as for
Unity?
Can you run system-config-printer --debug from a terminal and attach that here.
Does the troubleshooting wizard tell you something:
https://wiki.ubuntu.com/DebuggingPrintingProblems#Troubleshooting_Wizard
** Changed in: system-config-printer (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/987429
Title:
cannot add printer
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