UPDATE: Issue Resolved – Root Cause Found (Corrupted Steam .TMP files)
I finally managed to fix the crash! Even after cleaning my main Steam installation, the error returned as soon as I re-added my secondary library (Drive E:), which contains over 1TB of games and had recently suffered a hardware/file system error.
The Culprit:
The crash was caused by leftover temporary manifest files in the SteamLibrary\steamapps\ folder. In my case, I found 195 files with extensions like .acf.tmp or appmanifest_xxxxx.acf.xxxx.
When a disk failure occurs during a Steam update, Steam leaves these partial "shadow" files behind. While Steam itself ignores them, LaunchBox’s startup scan (specifically the GetInstalledAppIds method) appears to attempt to parse any file beginning with "appmanifest". Because these .tmp files contain binary "garbage" or null characters instead of proper text, the .NET 9 runtime in LaunchBox 13.24 fails to calculate the string length, triggering the error
ArgumentOutOfRangeException: minimumLength ('-2147483064')
How to Fix:
If you have a large library and LaunchBox crashes only when your Steam drive is connected:
Go to your Steam library folder (e.g., E:\SteamLibrary\steamapps\).
Search for *.acf.* or look for any file that does not end exactly in .acf.
Delete all files ending in .tmp or ending with a string of numbers (e.g., appmanifest_12345.acf.6789).
Ensure only the clean .acf files remain.
Also, clear the steamapps\downloading folder on that drive, as it may contain similar corrupted data.
Once I deleted those 195 corrupted temporary files, LaunchBox opened instantly without any errors, and all my platforms loaded perfectly.
Conclusion:
If your drive has ever had a "dirty" shutdown or a disk error, check for these hidden .tmp manifest files. .NET 9 is extremely sensitive to the malformed data inside them, and cleaning them up is the only way to stop the crash.
AliMujahid20's post in Can't start LB, problem related to Steam storefront was marked as the answer