Multiple invoices that were paid last year were re-opened and are now stated as overdue. I dont know why this happened.

I'll share some details about invoice payments in QuickBooks Online, @mario-foreignper.

It could be that the invoice payment was unlink since the invoice was re-opened. To fix this, if you've received the payment as a deposit, you can edit it to link it with your customer's invoice.

Here's how: 

1. Go to the Accounting menu.
2. Choose the Chart of Accounts.
3. Locate the account you deposited the payment to, and then click View Register.
4. Find and choose the correct deposit, select Edit.
5. On the Bank Deposit screen, scroll down to Add funds to this deposit, then find the fund you need to edit.
6. In the Received From column, select the appropriate customer name.
7. In the Account column, search and choose Account's Receivable.
8. Once done, click Save and Close.

Once the Deposit entry is updated, apply it as a credit to the invoice.

1. Click the + New. button.
2. Under Customers, choose Receive Payment.
3. Select the Customer's name with all open customer invoices and outstanding credits.
4. Add the Payment date.
5. Under Outstanding Transactions, choose the invoice you need to mark as paid.
6. Under Credits, select the Deposit entry.
7. Once done, select Save and Close.

For more information, feel free to check out these articles: 

• How to Link Deposit to an Invoice?
• How to Record a Customer Payment?

Additionally, I've added this link to guide record bank deposits in QuickBooks: Enter and make bank deposits.

Know that I’m always around to help if you have questions in managing your invoices. It’s always my pleasure to help you. Have a good one.

Any "adjustments" made to the checking account that affected the payments? Adjustments made preparing for taxes or any other Year End adjustments?  Changes in the associated Chart of Accounts? 

Go back to the invoices and the payments and make sure they're still linked and didn't get altered somehow.  (Wouldn't be the first time.) 

Hi, @mario-foreignper.

Hope you’re doing great. I wanted to see how everything is going about your concern with the invoices you had the other day. Was it resolved? Do you need any additional help or clarification? If you do, just let me know. I’d be happy to help you at any time.

Looking forward to your reply. Have a pleasant day ahead!