Hello there! @Walck. Welcome to QuickBooks Community! I'd be happy to share some details about incorrect check numbers.

There are times that your browser is full of frequently-accessed page resources causing unusual behavior. You can do these troubleshooting actions to check if the issue is browser-related.

Here’s how: 
   

   1. Restart your browser. 

   2. Use other browser like Incognito.

   3. Clear your browser’s cache and browsing history.

Otherwise, you can check your settings to see if this option is on: Warn if duplicate check number is used.

You can do so by following the steps below: 
   

  1. Go to the Gear icon.

  2. Select Account and Settings.

  3. Click Advanced tab.

  4. Look for Other preferences, then hit the pencil icon.

  5. Tick the box for Warn if duplicated check number is used.

  6. Click Save, then Done.

If issue still persist after following the steps above, I would advise to reach out to our QuickBooks Online Support and have them check your account to see what’s really causing this issue.

Feel free to reach out if you have any further questions. Have a good one!