Android client unable to find server, but server is up and running

Yesterday my Android client stopped uploading files, issuing “server not found” messages. However, my server is up and running all right. I restarted the phone, but it did not help. I can connect to my server from the phone (using the browser, using a network analyzer app), so it does not seem to be a phone-related issue (with DNS resolution, for example).

This setup used to work for several years, now it’s boken, what could be going on?