When using any one of the "Search by..." tabs, Directory Finder will Start and find nothing.
If you also check "Look only for directories named" but do not provide an argument, Directory Finder will Start and find nothing.
If you do provide an argument (such as *), Directory Finder finds what you asked for in a "Search by..." tab.
Conclusion: the Start button should be greyed out until sufficient information is provided.
Fixed in PT v. 1.5.2.336