Please try to change your ISP (internet service provider), as others have suggested on this thread. If you ever encounter the same problem again, feel free to reach out to us here at any time.
I have downloaded a VPN specifically Express VPN and it works like a charm.
Works perfectly fine and now i can finally read my messages and reply to them.
It costs $99 for the year but i just paid the $12 for the month to try it out.
In case someone still has this issue: what helped me was to install beta version of Upwork (this version won't even load because it says "no internet connection" - when I click "check connection", everything shows up okay though).
Then I didn't uninstall this version, but installed the regular version right away. After this, everything worked all of a sudden.