I'm working with older hardware, so my setup is Linux Mint 22 (Ubuntu 24.04 base), Cinnamon on X11, NVIDIA RTX 2060 (driver 595.91.07), umu-launcher 1.4.4 (.deb from GitHub), and the x86_64 launcher AppImage that corresponds to my base version.

1. Pressing Play hangs because umu-run crashes on startup
The AppImage sets PYTHONHOME and PYTHONPATH to its own mount (/tmp/.mount_*/usr/), and umu-run inherits them. Since umu-run is a Python script using the system Python, it dies immediately with:
Fatal Python error: init_fs_encoding ... ModuleNotFoundError: No module named 'encodings'
The launcher doesn't notice the crash and just sits there.

LD_LIBRARY_PATH leaks the same way. umu folds it into STEAM_RUNTIME_LIBRARY_PATH, so the game itself ends up loading the launcher's bundled libraries. I saw Failed to load module: /tmp/.mount_*/…/libgiognutls.so: undefined symbol, and one launch died at startup with X Error … GLX … X_GLXCreateContext.

Workaround: a wrapper at ~/.local/bin/umu-run that unsets PYTHONHOME PYTHONPATH LD_LIBRARY_PATH GIO_EXTRA_MODULES GSETTINGS_SCHEMA_DIR GTK_PATH GTK_EXE_PREFIX GTK_DATA_PREFIX GTK_IM_MODULE_FILE GDK_PIXBUF_MODULE_FILE GST_PLUGIN_SYSTEM_PATH GST_PLUGIN_SYSTEM_PATH_1_0 QT_PLUGIN_PATH PERLLIB before calling /usr/bin/umu-run.
Suggested fix: clear the AppImage-specific variables before spawning umu-run.
Note: This was was mostly solved via LLM, so YMMV.

2. Launcher sometimes fails to open on NVIDIA
Could not create GBM EGL display: EGL_NOT_INITIALIZED. Aborting...
Fixed by launching with WEBKIT_DISABLE_DMABUF_RENDERER=1 (a known WebKitGTK + NVIDIA issue). It's intermittent: earlier launches worked fine.
Note: This one was solved entirely via LLM, so YMMV.

3. Periodic freezes with GE-Proton11-7
With the GE-Proton the launcher requests, I got 1–1.7 second freezes every few seconds, even on the server list. GPU and CPU usage were low, and disconnecting from the network made no difference. Weirdly, this didn't happen during the first launch of the game (only intermittent hitching which calmed down after a few minutes). Subsequent launches were entirely unplayable due to the freezing. After trying a few settings, I found switching umu to Valve's Proton 11.0 (by setting PROTONPATH in the wrapper from #1) fixed almost all of it. Running windowed with the compositor on made it worse and conversely running full screen without the compositor became waaaaay better. Also noticed that it seems like raising the framerate target raises the likelihood for a hitch, but even with 120fps on my old hardware, it's well within the playable range.

Final notes: I'm both new to Linux and especially Linux gaming so I apologize if this isn't like how all this is supposed to be done. I just figured putting this on the forum might help with someone else searching for similar issues or if they were asking an agent or just searching for issues by keywords.