Did you find a solution?
I'm experiencing more or less the same problem: I'm not able to perform the logout from the external portal page even if we have an html button (a submit form) which point to the suggest URL:
https://securelogin.arubanetworks.com/auth/logout.html
I can reach the logout.html page but the AP does not perform the logout (it does not send the RADIUS Acct-Stop message and the wifi client can continue to reach Internet).
I'm using one AP 105 in Instant mode; the firmware is the latest:
Build date: 2019-03-10 20:32:06 PDT
Version: 6.4.4.8-4.2.4.13_69505
I've seen that the suggested logout URL was posted 4 years ago; maybe something is changed in the exposed API?
Thanks,
Alberto