Assuming you are on some Chrome/Chromium or Firefox browser you should get the web developer tools if you hit F12 and both of those have a Network tab. There it shows all the HTTP requests your browser makes from that point on. Try to open that before you try to reproduce the problem again.

The network view has a table of requests to your lemmy instance’s server. A new one will likely appear at a time just before that error message pops up. Click the rows one by one. Every time you click a row a detail panel for that row will show you more details. You want to check the response body. It should start with what you see in your error message instead of most likely starting with an opening curly brace for JSON.