kwin_x11 refuse to start, resulting in unusable GUI

[marius851000]

Describe the bug
After an update (coinciding with me switching to flake for my system), I had multiple error:
sddm doesn't display the correct theme (seem to use the default one)
kwin_x11 refuse to start (see the log for when I run it manually)

To Reproduce
Steps to reproduce the behavior:

1. git clone https://framagit.org/marius851000/nix-home-config.git
2. git checkout notworking
3. cd root
4. copy your hardware-configuration.nix in this folder
5. sudo nixos-rebuild switch --flake $PWD#marius-tuf --upgrade -v
6. maybe reboot

Expected behavior
sddm have the same style that usual, and kwin_x11 start, allowing to use the GUI.

Additional context
here is the log of kwin_x11 before exiting:

/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libkdeinit5_kwin_x11.so)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5ConfigWidgets.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5Auth.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5AuthCore.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5WidgetsAddons.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libkdecorations2.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libkdecorations2private.so.6)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKScreenLocker.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /nix/store/zkpivzapj1vb0pkdnvsis9w4d5mlkr3z-qtdeclarative-5.14.2/lib/libQt5QmlModels.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5KIOWidgets.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5KIOGui.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5KIOCore.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /run/current-system/sw/lib/libKF5GuiAddons.so.5)
/home/marius/.nix-profile/bin/kwin_x11: /nix/store/sq9g6x30xzhhl6fdz2qa98wnhvbg1lhb-qtbase-5.12.7/lib/libQt5Core.so.5: version `Qt_5.14' not found (required by /nix/store/675a51vhk3mll3qjbsmcy24kf1ixpsq6-kidletime-5.71.0/lib/libKF5IdleTime.so.5)

Also, on the master branch (that I use), I found a commit that work (but had problem running a bisection).

Notify maintainers
@ttuegel

Metadata

• system: "x86_64-linux"
• host os: Linux 5.7.12, NixOS, 20.09.20200805.6a0fd33 (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20200721_ff314f1
• channels(root): "nixos-20.09pre238658.2e40e4578fd"
• channels(marius): "home-manager, mariusnur"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: plasma5.kwin
# a list of nixos modules affected by the problem
module:

[ttuegel]

This error is caused by mixing different Qt major versions between your system and user profiles, with an old version of Plasma (and Qt) installed in the user profile. This is a common problem, and for this reason installing Plasma in the user profile is not a supported configuration.

[marius851000]

Thank. This solved my problem.