Android 17 isn’t the problem. Your Plex server is reachable, but both the server and plex.tv reject the sign-in Symfonium has saved (“User could not be authenticated”). The app keeps retrying for about a minute and a half and then gives up, which is why it looks like a timeout.
This usually happens when the Plex sign-in has expired or been revoked, for example after a Plex password change or “sign out of all devices”.
To fix it, edit your Plex media provider in Symfonium. In the Authentication section, press Sign-In to generate a new token, finish the plex.tv sign-in, save, then run a sync again. If you use a home user with a PIN, make sure the PIN is still correct.
If sync still fails afterwards, please upload a fresh log right after the failed sync.