Sometimes, unexpected errors are caused by the stored cache data in your browser. Let's access your account via an incognito window to see if it's browser-related or not.
Press CTRL+SHIFT+N for Chrome.
Hold CTRL+SHIFT+P for Mozilla Firefox.
Once done, go back and check if you're still getting the same error. If it's working fine, you can clear the cache of your regular browser. Doing this will delete the temporary data files that are causing errors as you have.
In addition, using a different browser can be an option, too. Please contact our Customer Care Support if you're getting the same error. This way, we can check your account in a secure environment and investigate further.
I'll also share with you our QuickBooks Online page in case you'll need some related articles for your reference.
Let me know if you need help with anything else. I'll be around to assist you every step of the way. Take care and have a wonderful day ahead.