Thanks for reporting and investigating this, @quorm. I’ve logged this bug report for tracking this issue: Ulysses integration not working for some users - Bug Reports - Hook Productivity Forum.
They acknowledged the integration in a Tweet, but have not indicated to me that they are using Hook. Now that there Beta is over, I assume/hope they’ll be able to look at Hook.
changing the title, but tracking it with a separate bug report will be easier (more focused) and Ulysses users will hopefully find it in the Bug Report section and hopefully contribute to the topic.