ok - so I can't back out from the player once it gets caught in the login loop, as there's no user/pass to enter, so I can't get back to the main menu to get at the debug logfile, not without exiting via the firesticks home button and then re-entering VLC. Which means that log keeps disappearing before I can get to it. However, 3.5.3 works fine on the firestick for accessing my two servers (one for video - win10 - and one for music - win11). Uninstall VLC first, then I used the "download" app to download from:
https://get.videolan.org/vlc-android/3.5.3/
and get this file:
VLC-Android-3.5.3-armeabi-v7a.apk
Installing that and then going to the main app section on the firestick and turning off auto-updates should cure this until it's fixed.