I tried in lab, with a 26kB PNG file, and in the preview I can see the uploaded logo after a save/commit changes. I tested on Mobility Conductor, but would expect the same on a standalone controller. It's not easy for me now to test with an actual captive portal authentication.
I would not expect your image to be much larger (in size), you may try to change the filename to be shorter and/or get the _ removed, just use lowercase alpha-numerical in the filename. In AOS8, some fields (like roles) are made non-case-sensitive, and the logo filename may not be aware of that. It may also provide some insights to use the developer tools in your webbrowser and find out what is the filename that's attempted to be retrieved.
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Herman Robers
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If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check
https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.
In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
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