Occasionally you may see a red X warning show up next to your filter on the enhanced search filter, which indicates that there is an issue with the filter. The screenshot below shows an example of what the warning triangle looks like.
When you see this warning triangle then you can hover over the warning triangle in order to see further information on what is causing the warning.
If hovering over the filter does not indicate an issue related to missing fields or projects then this could indicate that the filter needs to be resynchronized. You can resynchronize the filter by running the search again and clicking the Sync Filter button in order to see if the warning is removed. Refer to the Get Help section if you continue to see warnings with your filter.