action_err_ns on saving pages

On a site running Revo 2.3.3 I am getting an error message of "action_err_ns" in a message box when I save resources.

Additionally, it seems the generate this error (often multiple times) in the error log:

[timestamp] (ERROR @ path/model/modx/modx.class.php : 703) PHP warning: Invalid argument supplied for foreach()

Despite the error, the resources actually do seem to be saved properly, (although the Manager thinks they are not and asks if you really want to abandon your changes when you navigate to another page; plus you have to clear cache manually to make the changes take effect).

This seems to have no negative impact but obviously something is not quite right. This didn't used to happen and I can't identify any change that could have caused it. I also can't identify any pattern as to whether specific types of resource/setting/template affect it.

Has anyone else seem this or have any idea what might be causing it?

Thanks.