In some cases when you update an old 2sxc site you may run into this error message in the UI for string-wysiwyg fields and probably also for custom-gps fields. 

Field Configuration is Incomplete and should be fixed

Cause

This is complicated and historic, so let's quickly explain.

• Before 2sxc 10 the configuration for input-fields was stored in the SQL Database. In v10 we changed this to be JSON files but older installations still have the configuration stored
• Normally the configuration was stored in App #1 which was not used except for system data. The default App which the first Content will get is normally #2
• Now the rule in picking up Input-Field configurations is to check if the current App has any configurations (overriding the defaults or adding additional fields). Since normally App #2 has no configuration stored, it works. 
• Now in rare cases installations which started very long ago put the Content in App #1. This doesn't usually matter, but in this case it will check and find a definition in the current App and prioritize it. 
• Since the definition for string-wysiwyg changed for V10+, it's now missing data.

Verify this is your Issue

To verify that this is the cause, check that your call (F12) is actually trying to edit data on App #1:

Fixing this Issue

The goal is to deactivate the relevant settings in App 1. You could just remove all, but let's play safe. 

1. Go to App Data and switch scopes to Fields
   
2. Go to the Custom Input Type Configuration list
   
3. Find the definition for string-wysiwyg and delete or edit it - like just add a few xxxs
   When multiple languages are installed delete it for sure.
   

Everything should now work.