I was able to get into the prefab editor by bypassing the button and using this command in powershell:
& "S:\SteamLibrary\steamapps\common\Arma Reforger Tools\Workbench\ArmaReforgerWorkbenchSteamDiag.exe" -wbModule=WorldEditor -run -load "C:\Users\USER\Documents\My Games\ArmaReforgerWorkbench\addons\Crash Test\Prefabs\Vehicles\Wheeled\BRDM2\BRDM2.et"

But when attempting to open it using "Edit Prefab" the issue comes back

Workaround found:

I believe I have found a reliable workaround and some additional information that helps narrow down the issue.

The crash consistently occurred when using Edit Prefab normally. The viewport would briefly/fully turn black and Workbench would then crash.

I discovered that I could bypass the crash by launching Workbench directly into World Editor with a random prefab:

& "S:\SteamLibrary\steamapps\common\Arma Reforger Tools\Workbench\ArmaReforgerWorkbenchSteamDiag.exe" -wbModule=WorldEditor -run -load "C:\Users\USER\Documents\My Games\ArmaReforgerWorkbench\addons\Crash Test\Prefabs\Vehicles\Wheeled\BRDM2\BRDM2.et"

This successfully opened the prefab in Prefab Edit Mode.

Interestingly, simply opening it this way was not enough. After closing Workbench, the normal Edit Prefab action still crashed.

However, I then reopened the prefab using the same command, made a small change to the prefab, and saved it.

After doing that, I closed and reopened Workbench, and the normal Edit Prefab action started working again. I have since tested multiple prefabs and they now enter Edit Prefab mode normally.

Reproduction / workaround

1. Normal Edit Prefab → crash.

2. Launch directly with -wbModule=WorldEditor -run -load <prefab>.

3. Prefab opens correctly in Prefab Edit Mode.

4. Make any change and save the prefab.

5. Restart Workbench.

6. Normal Edit Prefab now works again, including for other prefabs.

This makes me suspect some prefab/editor state, generated metadata, or initialization data was missing or stale, and successfully entering and saving a prefab through the direct World Editor path initialized/rebuilt whatever was required.

The original crash was an access violation in Qt6Gui.dll during QPlainTextEdit painting, but the black viewport occurring immediately before the crash suggests the Qt crash may have been a secondary symptom.

The issue appears to be resolved on my installation after performing the above workaround, but the fact that a direct World Editor launch + save can restore normal Edit Prefab functionality may be useful for identifying the underlying bug.

I have the exact same issue. Hope a better fix is found so tools is normally able to open prefab editor