List Fields attempt to do a so-called high quality rendering of the rich text fields in the selected Notes documents. This is done with the Domino XML Language (DXL). Unfortunately, it's not possible to detach any attachments during DXL processing.
If the document contain large- and/or many attachments, DXL may be occupied with data processing when you try to save or open an attachment. Unfortunately the Notes C/C++ API won't allow more than one active application-thread accessing the Notes database at any given time.
This message informs you about this issue, and you may wait and retry the save or open action again later.
You may also control the so-called size thresholds that List Fields use to determine if it should process documents with DXL at all. In other words, if the document contains too large- or too many attachments, you may hinder List Fields from processing the document with DXL. The disadvantage with this option is that you won't be able to see the high quality rendering of rich text fields. The advantage is that you may save or open your attachments quicker! You can manipulate the size threshold in the Preferences dialog box.