In v1.5.8, the "Add a Comment to the Action List" action was changed to allow blank input, so that empty lines could be used as spacers.
However, if the contents of the "Comment" textbox are changed in any way, either when editing or creating the action, this functionality is lost; Removing text and going back to blank input will result in "OK" being greyed out.
Inserting a space does not remedy this, presumably this is because spaces are trimmed, which would result in blank input.
EDIT: Working in v1.6.7.8