I've got you from here. I'm committed to getting you back up and running, because I want you to be able to use QuickBooks to it's fullest. You mentioned that you had already been on hold with Customer Service. I hope they were able to assist you, but just in case, I'll go over some steps I believe will help you with this. This issue could be stemming from how the server is set up for Multi-User Mode. Here's how you can check that:
Note: these steps have to be performed on the computer hosting the company file.
If you're still having issues, I'm attaching an article with additional troubleshooting steps: https://community.intuit.com/articles/1502150-error-h101-h202-h303-or-h505-when-opening-your-company... . If you feel uncomfortable performing any of these steps, feel free to reach out to me, or an IT professional. I hope this helps. Keep me posted on your progress.