TYPO invalid magic
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Nevenmind (I could not delete this post). I saw a bug for my version of Pfsense. I will upgrade the hardware+software, so I will think this problem goes away.
Pfsense version: 2.4.2
just for information,
HA has been really, really improved for the captive portal in the next pfsense version (which is currently not released). There is now an active-backup mechanism for keeping connected users in case of a failure; vouchers are better synchronized; etc...
if your problem doesn't go away with an update, you can update your pfsense appliances to the developpment version
@free4 thank you for the info! I will look if I can find a release date. Perhaps it is worth to wait a bit with the upgrade