I saved the pdf in archival format (PDF/A); that would not upload either.
What worked was creating the Research entry and then adding the document to it. There was a different error the first time I tried, but it worked the second time.
I found a solution. First I uploaded my paper to the academia, then I downloaded the file I uploaded to the academia. Finally, I tried to upload this file to researchgate and I succeeded.
Resolution: I tried to insert a book and encountered the same situation. After many attempts, I managed to resolve the issue in the following way:
1 - I manually entered the book data without uploading the PDF.
2 - After publishing the data, I went to "edit" and uploaded the PDF. Even though the error message will appear, the PDF will be added successfully and made public.
3 - After the error message, just go back to your profile and check the availability of the work.