"invalid command" has nothing to do with any particular field so adding/removing fields are unlikely to change your results.
Is the file hosted on a server or still local on your machine?
Save a Copy As does not exactly save container data, it saves a complete new copy of your file where the container data is now embedded in the container fields. Did you specify a file name for the new copy of this file?
Are you doing this manually or in a script? If you are using a script, try doing it manually.
I got it working. I ran the script Flush Cache and then restarted Filemaker.