Try to reinstall your QB program. QB 2021 has been discontinued since June 2024. You should not have received any updates since then.

You'll need to reinstall your QuickBooks program and update your QuickBooks server, abconstr.

SIAB is correct, this issue is not related to Intuit's system since your QuickBooks Desktop (QBDT) version 2021 has been discontinued and should not have received any updates.

Before reinstalling, I recommend creating a backup copy of your company file. Then, you can reinstall your QBDT program. Once done, update your QuickBooks server. Here's how:

1. Go to QuickBooks [year] Server next to the Apple logo.
2. Select Check for Quickbooks Server updates.
3. Click Install Update.
4. Hit Install and Relaunch.

For more information, visit this article: Use multiuser mode in QuickBooks Desktop for Mac.

If you have additional questions, don't hesitate to leave a comment below.

This is bad advice. The issue is that the ssl certificate expired for Quickbooks Mac 2021 two days ago. To fix the issue Quickbooks would have to release a patch with a new certificate, which they likely will not do. The only fix is to move the company file from the server to the local Mac and open from there. 

Hello @abconstr you should be able to go to the following site and download the latest R15 version of QB Mac 2021, it will contain the full installer and the Server application that will work with it.  You just need to select the US for the country, QuickBooks Mac Desktop for the product and 2021 for the version.  Hopefully this will fix your issue.

https://downloads.quickbooks.com/app/qbdt/products?src=care

I'm having the same problem as well. I thought it was my older computers and so I moved everything to new computers, fully updated and downloaded the new server. Still won't work. This is extremely frustrating!

Just started having the same problem yesterday, 8/4. Moved everything to new computers, downloaded QB21 fresh and still having the same problem. From what I'm finding is that they need to renew the certificate. I don't know if that will happen!

@hilltopct 

Install the trial version of 2023 or 2024 to isolate the problem. Did you encounter the same problem?

After digging into this locally, the certificate has expired causing the problem.  Unfortunately with QuickBooks Mac 2021 no longer being supported an update for this most likely will not be released, so multi user access will not work anymore for this version.  

Seems like it should be possible to grab certificates from a more recent downloadable version, and replace the QB 2021 certificates (assuming these are not hard-coded/compiled-in for some reason).

@futuit 

You can use the trial version of 2023 or 2024 as a workaround.

How long would the trial versions run without trouble?

Would be nice if you could connect to a newer QB Server version with your perpetual 2021 Desktop, but I doubt that works.

Related info for those of us being forced to consider an upgrade:

https://www.reddit.com/r/QuickBooks/comments/1lr2jky/stacksocial_scammed_me_with_a_fake_quickbooks_m...

Seems Intuit no longer offers a lifetime Desktop license, and the last version was QB 2021 (or perhaps 2022?)

Anyone have more details?

How many user license do you have now? Only single user or more?