I've checked from our records and found an ongoing investigation about the error message you're getting. Our engineers are now investigating the error message you are experiencing when merging customers or vendors in QuickBooks Online.
At this time, I encourage reaching out to our Support Team so you'll be added to the list of affected users in this investigation (INV-45874). Once added, you'll automatically receive an email notification with the latest updates.
To contact support:
Post again if you have any other questions. I'm always here to help. Have a good day!
I appreciate the time and effort you've spent with one of our phone support agents, ajwright.
Our engineers are still working and investigating to identify the root cause of this issue for them to apply for a permanent resolution. For now, we don't have a specific time frame for when is this going to be mitigated. Rest assured, this is being taken care of.
Since you contacted our Customer Support Team, you are already added to the list of affected users. Then, they'll send you an email notification once there's an update of this case.
I've got a link here that provides you with articles about managing your customer's transactions: https://quickbooks.intuit.com/learn-support/en-us/sales-and-customers/07?product=QuickBooks%20Online.
We appreciate your patience in this matter. I'll also keep an eye on the issue and will get back to you if there's an update. Keep safe and healthy.
Thanks for joining on this thread, @AWC FAMILY.
For now, the issue when merging customers (Error-3000) is still ongoing. Our engineers are doing their best to provide a permanent fix to this.
If you haven't contacted our QBO Support yet, I'd recommend reaching out to them. This way, they can add your account details to the list of affected users and notify you by email as soon as a fix is implemented.
Here's how you can reach out to them:
Please check out our support hours to ensure that we address your concerns on time.
Feel free to go back to this thread if have further questions. I'll always have your back.