I finally got my app working again (version 25.3.12 on Windows 11). Symptom was that the app simply failed to start with no visible activity whatsoever (no window opened). Uninstalling, reinstalling, downgrading to 24.xx didn't help at all.
I traced the problem back to a corrupt install of the Microsoft C++ redistributable (basically a common piece of code supplied by Microsoft and installed automatically). You can tell if this is the issue by checking the event log. First try and run the DB manager and let it fail to do anything. Then click "Start", type "event", and open the "Event Viewer". Look at Windows Logs, Application. You may see an "Error" type, source "Application error". If the details mention something like this:
Faulting application name: Garmin Aviation Database Manager.exe, version: 24.12.4.0, time stamp: 0x664f9114 Faulting module name: MSVCP140.dll, version: 14.34.31931.0, time stamp: 0x0ef31724
Then this will fix you. Click the Start button and search "add or remove programs" and open it. Search "redistributable" and pick the 2015-2022 (x86) version, click the three dots to the right, and choose "modify", then choose "repair". Once it's done, the DB manager should work!
