The old version (1.4.2 -) does not check if more than two encription keys in a database,but the new version (1.4.2) will check this and allows only unique key value.
In order to ensure that the key values in all fields is consistent,when the current database can not match the encryption key in the menu's Settings page,you will not be able to add or modify any items with encrypted fields .
You can try to export all templates to csv files and copy to your computer,then check if any fields displayed as "復号化中にエラーが発生しました。" in Microsoft Excel,change these values to ""(blank).
Next please refer to the following figure...
Copy these files to your PC
Open these files from your Microsoft Excel (separate by comma)
Change these error values to ""(blank).
Save these files as the same names(by UTF-8 format) and copy back to your mobile.
Preview these files,if they are correct,then next ...
Backup your database (Maybe you want to restore your database and redo in the future)
Delete all templates
Go to the [Settings Page] ,the Encryption Key is still "失敗".
Change the key to other values as you like ,then press the "OK" and "再暗号化" button
Now the Encription Key is Successful !
Import all csv files as new templates
Now,reset the important field as an encryption field