Have you tried disabling the Nik filter folder to see if that it the problem?
Are the Nik filters updated for CS6?
Not sure I know what you mean by "disabling the Nik filter folder".
I have the latest updates for the Nik filters and they are compatible with CS6.
Since yesterday I tried uninstalling the Nik filters, manually locating and removing every file related to Nik/Nik Software, redownloading the Nik installers, uninstalling/reinstalling Photoshop, and reinstalling the Nik filters. Same result.
I am pretty sure this is something on the Photoshop end, but am not sure what to do next. Maybe repeat the above and use the Creative Suite cleaner before reinstalling?
No, it means that there is a bug in the Nik plugins, and you need to talk to Nik.
They might have an update, or they might still be working on fixes to their plugins.
Thoughtful and helpful answer, Chris, thanks for that.
If it is a bug in the Nik plugins why did they work perfectly before I had to uninstall/reinstall Photoshop? In addition, the error message fails two of the three tests of a good UI element: while it is clear why I care, it is not clear what (specifically) the error means and what I should do to remedy it.
I don't know, since I don't know the nature of the bug in Nik's code.
They have had problems in the past due to just small OS updates.
The error message tells you what happened (an error in the plugin interface), as much as is known - and Photoshop doesn't know what the specific error was in the plugin, so has no idea how to tell you to fix it.
while it is clear why I care, it is not clear what (specifically) the error means and what I should do to remedy it.
A plug-in can choose to put up its own error messages programmatically, and make them as verbose as it likes, but it generally gets to provide only limited information back to Photoshop. I believe the message you're seeing occurs due to the plug-in returning code "paramErr" back to Photoshop.
I now get the same message with all of the DXO Nik plug-ins. Everything works fine with LRCC Classic, but nothing works with PSCC 2018 (19.1.4). I have sent support requests to both DXO and Adobe.