Hello there, @gtschirgi.

Thank you for taking the time posting in the Community. Allow me to help walk you through in resetting the check numbering in your QuickBooks Online. 

There's no set up for check numbering in your account. You can customize the number to whatever you prefer. 

To correct this, first, you need to assign one check number. After that, QuickBooks will increment the number by one when you create another check, or the rest should generally follow in order.

If you need additional help, feel free to reach out to our Customer Care support. The support agents will be able to assist you in getting this resolved.

Here's how you can reach them: 

1. Sign in to your QuickBooks Online account. 
2. At the top right, click the Help icon. 
3. At the bottom, click Contact Us. 

That should get you back on track.

I'm just a post away if you should have any other questions about resetting the check numbering in QuickBooks. I'll be happy to help you out.  Have a great rest of your week.