I encountered this crash playing the FTBInferno modpack using the flatpak version of ATLauncher. After some testing, I narrowed down the issue to the call to FileUtil.isPathPortable(p) in CapsuleTemplateManager.resolvePath(ResourceLocation locationIn, String extIn). Since the flatpak's absolute path includes ".../com.atlauncher.ATLauncher/...", the "COM" prefix is flagged by the regex comparison for RESERVED_WINDOWS_FILENAMES in FileUtil.isPathPortable and the exception is triggered despite this being a perfectly valid Linux filepath.
Full Report: crash-2026-08-06_15.01.21-server.txt
I encountered this crash playing the FTBInferno modpack using the flatpak version of ATLauncher. After some testing, I narrowed down the issue to the call to
FileUtil.isPathPortable(p)inCapsuleTemplateManager.resolvePath(ResourceLocation locationIn, String extIn). Since the flatpak's absolute path includes ".../com.atlauncher.ATLauncher/...", the "COM" prefix is flagged by the regex comparison forRESERVED_WINDOWS_FILENAMESinFileUtil.isPathPortableand the exception is triggered despite this being a perfectly valid Linux filepath.Full Report: crash-2026-08-06_15.01.21-server.txt