I too used the "custom editor settings" link below the text editor control to change the settings from default to true and all but two of my errors in the event log went away. Turns out I had several texteditor controls in a DNNTabStrip and at some point during messing around with this, saved module level settings and instance level settings for one of the may instances. So here is a summary of where these settings are in the database.

The custom settings for the FCK Editor are stored in the tables HostSettings (for Portal wide settings) and TabModuleSettings for (Module and Instance settings). In HostSettings, the SettingName column starts with 'DNNFCKP' and in TabModuleSettings, it starts with 'DNNFCKM' for Module level settings and 'DNNFCKI' for Instance level settings. The individual settings seem to always be identified by three characters following a '#' symbol. For "HTML Encode output" and "Reverse HTML Encode output" they are #HEO and #RHE. For instance level settings, the control id of the specific editor control instance to which the settings apply will be appended to the end of the SettingName. The SQL to find them is below. If you are having this problem, the values of the SettingValue column is probably 'default'... it needs to be 'true' or 'false'. Oh, and I'm using DNN 5.1.4.

select * from HostSettings
where SettingName like 'DNNFCKP#%#HEO' or SettingName like 'DNNFCKP#%#RHE'

select * from TabModuleSettings
where SettingName like 'DNNFCK_#HEO%' or SettingName like 'DNNFCK_#RHE%'