This issue is currently on all platforms, I solved it two days ago on Windows by following this thread
OpenSubtitles.org has apparently updated their servers. That uncovered latent bugs in the very dumb HTTP support of the VLSub extension. To work around the problem (you might need administrator privileges): Go to the VLC installation directory. Look for the file named lua/extensions/VLSub.luac. Open that file in a binary or hexadecimal editor. (DO NOT USE a text editor.) Look for the character sequence "HTTP/1.1" (without quotes). Replace the last one by a zero: "HTTP/1.0". Repeat the procedure; there should be a second occurrence in the file. Save the file. (Re)start VLC.
However that solution won't work on Linux, so what to do?