I have a simple WebDirect layout with a button that has a script attached which performs the Open URL script step. When clicked on in Safari it works properly. If I view the same web page thru a FileMaker Web Viewer and click on the link it throws the yellow "Could not open window, please disable your browser pop-up blocker and try again." chunk. I verified in Safari settings that pop-ups are enabled for the website in use. FM Server is 17.0.2 on Windows. Client is Mac OS 10.13.6 with Safari 12.0.1.
I added a 2nd web view referencing a page on the same host with two links - one for another web site and a second with a target of BLANK. Both work in the web view even when safari prefs are set to default blocking.
Trying to figure out what's going on, I also built a little test app in xcode with a web view - when I click on the link there I get the yellow box but it also throws a dialog about "no application to open URL" -- the URL is a simple https URL. This makes me think that WebDirect is somehow trying to open an invalid URL which throws the blocker.