Hope you are doing well. Sorry for your experience with Acrobat Online Services and the delayed response.
Would you mind sharing the screenshot of the error you get so we can understand the issue better and assist you accordingly?
Also, do you experience the issue with all the files or with a specific file? If with a specific file, can you share the file with us for further investigation?
You can also view further troubleshooting steps from the given help article: Troubleshoot viewing PDF files on the web
Hope this helps.