Hi all,
I found a post from 5 years ago with the exact same problem and left a comment there but I thought maybe it would be easier if I made a post myself too.
So I did some troubleshooting and I came the conclusion that Panels handling auth is a bit weird. This is indeed a Panels unique issue. I can connect to my calibre OPDS server. I can connect to it on local network, and also remote via reverseproxy that is letsencrypt certified. In both cases, as soon as I tick the “Require username and password to access the Content server” option on “Sharing over the net”, connection times out. All OPDS servers I could find on the App Store (free ones, didnt want to pay to check for paid ones) connect to the OPDS server even with the credentials required, and it is only Panels 3 that doesnt work.
I just bought Panels 3 premium lifetime as I loved how the app looks and the panel by panel reading (havent tried it yet) seemed like a great feature. But I must say this is a bit frustrating that I cannot have a secure option to connect to my server with a login credential and other free apps can do it. Given that the first post that mentions this issue I could find in the forum dates 5 years back, I hope there is something I am missing and the team has figured out.