I attach screenshot that appears in the bottom right corner.

I don't know Croatian but according to Google Translate, the error message is telling you that the connection has timed out. When Thunderbird tries to retrieve your email it sends a request message to the server. If it doesn't get a response after a certain period of time it gives up and you get that error message. One possible cause (the most likely in my experience) is that the email server is down. Have you checked their website? Have you contacted them for help? Another possible cause is that your internet connection is not working or is very slow. Try connecting to different websites. At any rate this does not appear to be a problem with Thunderbird itself.

Ok, thank you very much, you were helpful. I will check with my we designer.