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UMG UI editor refuses to open

Suddenly, after saving and re-opening my project, the UMG editor refuses to open.
It doesn’t matter which widget blueprint I click; it just creates an invisible window that doesn’t open.
Here’s what it looks like in the task bar:

114f94403421e2cd069e1d94c5972ee5d201ad59.jpeg

Also, when opening the project, I get the following errors about one of the UI widgets; I don’t know if that’s related.
(Also this widget compiled and ran fine before saving, so where does this error come from?)

e88d9bf96e27089b676481622a5656eeb6a18b75.jpeg

What can I do to make the editor open UMG widgets properly again?

Btw: I’m using 4.8.3, because 4.9 refuses to build lighting (this is a known bug, apparently, and a bit of a blocker…)

More information: This also affects a new project; created from template; create new widget blueprint; double-click and it refuses to open (white inactive window visible in the taskbar.)
So, what kind of editor-level settings have gotten corrupted, and where can I fix them?

When I delete AppData\Local\UnrealEngine\4.8, I can now open up freshly blueprint widgets, although I then get a crash as follows:

Fatal error!

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0xffffffff

UE4Editor-CoreUObject.dll!UObjectBaseUtility::IsA() {0x00007ffc673124a4} + 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-SlateRHIRenderer.dll!FSlateRHIResourceManager::GetShaderResource() {0x00007ffc5e2c0376} + 56 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-SlateCore.dll!FSlateElementBatcher::AddBoxElement() {0x00007ffc668f4d5c} + 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-SlateCore.dll!FSlateElementBatcher::AddElements() {0x00007ffc668fa9e6} + 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-SlateRHIRenderer.dll!FSlateRHIRenderer::DrawWindows_Private() {0x00007ffc5e2b3a2d} + 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Slate.dll!FSlateApplication::PrivateDrawWindows() {0x00007ffc5d1f606b} + 116 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Slate.dll!FSlateApplication::DrawWindows() {0x00007ffc5d1aed40} + 13 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Slate.dll!FSlateApplication::Tick() {0x00007ffc5d21727f} + 8 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor.exe!FEngineLoop::Tick() {0x00007ff758384bf6} + 15 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor.exe!GuardedMain() {0x00007ff7583749cc} + 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor.exe!GuardedMainWrapper() {0x00007ff758374a4a} + 5 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor.exe!WinMain() {0x00007ff7583865c9} + 17 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor.exe!__tmainCRTStartup() {0x00007ff7583875d9} + 21 bytes [f:\dd\vctools\crt\crtw32\dllstuff\crtexe.c:618]
KERNEL32.DLL {0x00007ffc89b22d92} + 0 bytes
ntdll.dll {0x00007ffc8a419f64} + 0 bytes
ntdll.dll {0x00007ffc8a419f64} + 0 bytes