We have reports that filters are being intermittently rejected at times for invalid category IDs. This is an eBay issue likely related to a misconfigured or down server. We cannot resolve it at our end.
As a workaround, we have disabled the filter checking so that your filters will not be disabled when this occurs.
If you are not receiving items for a new filter, you will need to verify your category numbers.
You can also use more precise keyword filters and the All Channels option to avoid category searching altogether. This approach has the added benefit of not being impacted when eBay changes category numbers.