P.S. Because we are about to go live with this Toolset form, on my production site I have deactivated Relevanssi and removed the CRED Auto Draft posts. However, on the staging site (mindthegap.somo2.nl) Relevanssi is still active and the CRED Auto Draft are still visible.
Many thanks for investigating. I can't explain why the issue did not occur when you tested it. I have just done another 3 or 4 tests (with different attachments), and everytime it creates a CRED Autodraft Post.
Just to make sure we are testing with the same parameters:
1) Did you test it on the staging site (mindthegap.somo2.nl)? The issue will not occur on the production site (mindthegap.somo.nl), because I deactivated Relevanssi there.
2) Did you add the new post via the frontend, via this form: hidden link? It may not occur when adding posts via the backend.
3) And did you attach a document or image before submitting the new post? The problem does not occur when you add new posts without an attachment.
That is what I did yes, but I repeated the test this morning and I did manage to generate the auto-draft error.
I am taking a copy of your site so that I can do some more testing locally and in case I need to share it with our developers, and I'll update you again soon.
I installed the site locally, switched theme to twentyseventeen and disabled all non-Toolset plugins except Relevanssi and made some test submissions and was able to see the issue.
(I'm still not able to reproduce on my own test site.)
I'm escalating this and have passed a copy of your site on to my colleagues for analysis, and I'll update you when I get some feedback from them.