Hello!
Did you update the module to the latest version? This could maybe solve the problem.
I expect that you pass an object to the page where the PDF is generated?! Make sure to validate that the object isn't empty. Maybe in some scenarios this could be the issue.
Could anything else be using that object? I wonder if there is a lock on it so your action can't complete and times out.
While testing, I got the same problem as you. I pulled back the registration of the module in backend and created a new API key.
What I noticed with this is that an API Key had to be used before, and now it has to be a Personal Access Tokens (PAT).
From the documentation:
For registering your app environment(s), we now require a personal access token (PAT) instead of your developer API key. The PAT can be created in your Developer Settings in the Mendix Developer portal. For the scope, select 'mx:deployment:read' of the applicable deployment type. Existing app environment registrations stay valid after the module update, you do not need to re-register your app.
After changing this and upgrading to v1.5.0, it worked for me.