I cannot upload csv invoices, normally it is easily imported. But today it is written unknown error

We can perform the basic browser troubleshooting steps to fix the issue with uploading your invoices in QuickBooks Online (QBO), eduard. Let's get into the details.

Occasionally, QuickBooks may encounter glitches, such as importing issues due to excessive cached files in your browser. Let's go ahead with basic troubleshooting steps.

First, restart your browser. Then, log in to your account in a private browser and upload your invoices from there. You can use these keyboard shortcuts to launch a new private window:
 

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N
   

If it fixes the issue, go back to the regular browser and clear the cache. Doing this will eliminate outdated files or data that cause system issues and glitches. Otherwise, you can use another supported browser.

Once you've successfully imported your invoices in QuickBooks, you can check out this article for future reference in handling invoice payments: Record invoice payments in QuickBooks Online.

Any time you need support with importing invoices and other transactions in QuickBooks, I'm just a quick message away. Always happy to assist.

@eduardtedjobuwono 

Try opening your QBO account on any private/incognito browser.