I had the same error on Ubuntu 20.04 with an existing `.minecraft` directory. When I remove the directory and try again, I run into the `is not a valid directory` issue:
merlijn@howard:~/Downloads/Minecraft(1)/minecraft-launcher$ ./minecraft-launcher
[Info: 2021-03-11 16:51:02.309006261: NetQueue.cpp(157)] NetQueue: Setting up.
[Info: 2021-03-11 16:51:02.309302930: mainLinux.cpp(250)] Running launcher bootstrap (version 915)
[Info: 2021-03-11 16:51:02.309183842: NetQueue.cpp(575)] NetQueue: worker thread started.
[Debug: 2021-03-11 16:51:02.309589445: PlatformLinux.cpp(44)] HOME dir (getpwuid): /home/merlijn
[Debug: 2021-03-11 16:51:02.309601690: PlatformLinux.cpp(49)] HOME dir (env): /home/merlijn
[Error: 2021-03-11 16:51:02.310450485: Settings.cpp(40)] Failed to determine configuration file size: No such file or directory
[Info: 2021-03-11 16:51:02.310962141: Common.cpp(32)] Native Launcher Version: 915
[Info: 2021-03-11 16:51:02.310981386: Common.cpp(33)] Operating System: Linux
[Info: 2021-03-11 16:51:02.310994280: Common.cpp(34)] Application Data directory: //.../.minecraft
[Info: 2021-03-11 16:51:02.311009290: Common.cpp(35)] Executable Path: /.../minecraft-launcher
[Info: 2021-03-11 16:51:02.311020203: Common.cpp(36)] App Directory dir: //.../minecraft-launcher
[Info: 2021-03-11 16:51:02.311030579: Common.cpp(37)] Game data directory: //.../.minecraft
[Info: 2021-03-11 16:51:02.311040630: Common.cpp(38)] Launcher dir: //.../launcher
[Info: 2021-03-11 16:51:02.311053767: Common.cpp(39)] Java dir: //.../jre-x64
[Info: 2021-03-11 16:51:02.311064226: Common.cpp(40)] TmpDir dir: //.../MinecraftLauncher.1000
[Info: 2021-03-11 16:51:02.311074251: Common.cpp(41)] x64: true
[Info: 2021-03-11 16:51:02.311549053: mainLinux.cpp(308)] relaunchedExe: /home/merlijn/.minecraft/launcher/minecraft-launcher selfExe: /home/merlijn/Downloads/Minecraft(1)/minecraft-launcher/minecraft-launcher
[Info: 2021-03-11 16:51:02.311606914: mainLinux.cpp(312)] Top level executable, forwarding signals.
[Error: 2021-03-11 16:51:02.311648880: mainLinux.cpp(330)] Invalid start directory!
MessageBox(Minecraft Launcher): Cannot start Minecraft in "/home/merlijn/.minecraft": the specified path is not a valid directory.
[Info: 2021-03-11 16:51:02.312662503: NetQueue.cpp(585)] NetQueue: Starting net action [https://sentry.io/api/5254002/store/]
More info: MCL-17155
I'm not sure what I'm doing wrong but this is what I get when I run the latest minecraft launcher on Ubuntu 20.04.
merlijn@howard:~/Downloads/Minecraft(1)/minecraft-launcher$ ./minecraft-launcher
[Info: 2021-03-11 16:51:02.309006261: NetQueue.cpp(157)] NetQueue: Setting up.
[Info: 2021-03-11 16:51:02.309302930: mainLinux.cpp(250)] Running launcher bootstrap (version 915)
[Info: 2021-03-11 16:51:02.309183842: NetQueue.cpp(575)] NetQueue: worker thread started.
[Debug: 2021-03-11 16:51:02.309589445: PlatformLinux.cpp(44)] HOME dir (getpwuid): /home/merlijn
[Debug: 2021-03-11 16:51:02.309601690: PlatformLinux.cpp(49)] HOME dir (env): /home/merlijn
[Error: 2021-03-11 16:51:02.310450485: Settings.cpp(40)] Failed to determine configuration file size: No such file or directory
[Info: 2021-03-11 16:51:02.310962141: Common.cpp(32)] Native Launcher Version: 915
[Info: 2021-03-11 16:51:02.310981386: Common.cpp(33)] Operating System: Linux
[Info: 2021-03-11 16:51:02.310994280: Common.cpp(34)] Application Data directory: //.../.minecraft
[Info: 2021-03-11 16:51:02.311009290: Common.cpp(35)] Executable Path: /.../minecraft-launcher
[Info: 2021-03-11 16:51:02.311020203: Common.cpp(36)] App Directory dir: //.../minecraft-launcher
[Info: 2021-03-11 16:51:02.311030579: Common.cpp(37)] Game data directory: //.../.minecraft
[Info: 2021-03-11 16:51:02.311040630: Common.cpp(38)] Launcher dir: //.../launcher
[Info: 2021-03-11 16:51:02.311053767: Common.cpp(39)] Java dir: //.../jre-x64
[Info: 2021-03-11 16:51:02.311064226: Common.cpp(40)] TmpDir dir: //.../MinecraftLauncher.1000
[Info: 2021-03-11 16:51:02.311074251: Common.cpp(41)] x64: true
[Info: 2021-03-11 16:51:02.311549053: mainLinux.cpp(308)] relaunchedExe: /home/merlijn/.minecraft/launcher/minecraft-launcher selfExe: /home/merlijn/Downloads/Minecraft(1)/minecraft-launcher/minecraft-launcher
[Info: 2021-03-11 16:51:02.311606914: mainLinux.cpp(312)] Top level executable, forwarding signals.
[Error: 2021-03-11 16:51:02.311648880: mainLinux.cpp(330)] Invalid start directory!
MessageBox(Minecraft Launcher): Cannot start Minecraft in "/home/merlijn/.minecraft": the specified path is not a valid directory.
[Info: 2021-03-11 16:51:02.312662503: NetQueue.cpp(585)] NetQueue: Starting net action [https://sentry.io/api/5254002/store/]
This is even with a regular home directory so it looks like something regressed further.
Since this is a feature request, I can't upload a crash report
@AsciiWolf Until this issue is fixed, you can workaround it by using an LD_PRELOAD: https://github.com/ericpruitt/homeishome