I'm grateful for your efforts in performing the steps shared by my peers above, @orestys.
Let me get you pointed in the right direction to help isolate why the customer/vendor lists are not populating in QuickBooks Online. I'd suggest reaching out to our Customer Care Team. They have the tools that can investigate the root cause of this problem and apply a fix.
To do that, please follow the steps shared by my colleagues above. Know that our support hours are from Monday-Friday 6 AM to 6 PM PT and Saturday 6 AM to 3 PM PT.
For your reference, I'm also adding here some resources that you can scan through that can help you in managing vendors and customer's transactions in QBO:
Feel free to join us again here in the Community if you have any other concerns. We are just a post away!
