#macOS says it cannot verify the app
The build is not signed with a paid Apple certificate. Press Done, then open System Settings, Privacy & Security, scroll to Security and press Open Anyway beside Tessera. Only the first time. On macOS 14 and earlier, right-click Tessera and choose Open instead.
#Windows says it protected your PC
Same reason. Choose More info, then Run anyway. Only the first time.
#A pack is missing from search
Check Review: a pack waiting there is deliberately out of search until it has a license and a source. Check Archive too, and whether a filter is still on.
#Thumbnails are missing or gray
They are drawn in the background and cached. Settings, Previews and index, can build them for the whole library, retry the ones that failed, or clear them and start again. A format nothing can draw shows its file type instead.
#Something changed on disk and Tessera did not notice
It watches the library folder, but a big change made while it was closed can be missed. Settings, Previews and index, Read the library again reads every pack from scratch.
#A download will not fetch
Some pages need a browser to click through. Open the page yourself, copy the link to the file, and paste that instead. Sites Tessera knows are listed on the Downloads page.
#A backup or sync will not start
Both need a helper program: Kopia for backups, Syncthing for sync. Settings, Helpers, shows whether each is there, its version, and a button to fetch it again.
#Reporting a problem
Settings, Privacy and problems, Report a problem opens an issue with your system, the version and the recent log already filled in. Nothing is sent until you press send, and you can read exactly what it contains first.