I can think of two possible reasons why you're unable to add an accountant, aklassykut.

First, in QuickBooks Online adding an accountant requires admin rights. So make sure that you're logged in as Master or Company Admin.

Second, it could be a cache-related issue. Too much files may affect browsing performance. To clear it out, let's perform a few troubleshooting steps to resolve this.

Here's how:

1. Try logging in to a private window for this browsing type won't save any files.
2. If it works fine, go back to your main browser and clear its cache.
3. Alternatively, you can use other browser.

I have here a helpful article for your reference:  Invite or remove an accountant.

I'm always around to help if you have follow up questions.