Cloud libraries (iCloud Drive, Dropbox) showing nothing
Usually one of: you are signed out of iCloud Drive on the device, the folder you pointed Panels at is not actually inside iCloud Drive's sync location, iCloud has not finished downloading the files to this device yet (files on demand), or iCloud Drive is still indexing after a fresh install. Open Files.app, navigate to the folder you added, and confirm you can see your comics there. If you only see filenames with a cloud icon, wait for iCloud to finish downloading before re-adding the library in Panels.
OPDS server: connection fails or times out
Usually one of: the server URL is off by a path component (missing /opds), the wrong scheme (http vs https), the server is not reachable from outside your LAN and you are on cellular, the self signed certificate is not trusted, or the server is down. The fastest check: open the exact OPDS URL in Safari on the same device. If Safari cannot load it, Panels cannot either. For self signed certificates, Panels prompts you to trust the certificate on first connection. Make sure you tapped Accept.
OPDS server connects, but the library is empty or wrong
For Kavita, OPDS is disabled by default and must be enabled per account inside Kavita's user settings. Until you do that, the feed is real but empty. For Komga, the user account you signed in with may have restricted access to specific libraries. Check the account's library permissions in Komga's admin UI. Also confirm your OPDS URL points to the root of the feed and not to a single sub library.
Reading progress not syncing across my Apple devices
For Komga and Kavita, the server is the source of truth, so check the web UI first. If the web UI itself is out of sync between sessions, the problem is on the server side, not in Panels. For Calibre over OPDS, you must be signed in to a Panels account on every device, otherwise progress stays local to each device. For iCloud Drive cloud libraries, both devices must be signed into the same iCloud account and give iCloud Drive time to finish syncing; first sync after a fresh install can take a while.