Im in the same situation when trying to export. Except pressing enter does nothing but make a thud noise. Once I click export in the export filetype and size window, anytime I go back to file/export, nothing comes up anymore except a thud noise. Using a Retina 2012 15" MacBook Pro. Was wondering if maybe it had to be with me running OS 10.15 beta but who knows at this point.
Have the same thing shown in Affinity Photo