Created attachment 279561 [details]
This URL doesn't load in STP: https://workflowy.com. Instead, the Loading image is shown. The page opens without any issues on Safari that is shipped with OS X. All trackers-blockers are disabled.
Just tried the URL with SFTP 5 (Version 9.1.1 (11601.6.17, 11602.1.33)), the issue is resolved.