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Try this solution ... it worked for me
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I have found the solution with the help of previous hints. Just follow the steps below to resolve the issue:
1) Open folder of Start Menu containing shortcut to Acrobat Reader:
a) Select shortcut in Start Menu.
b) Right click on shortcut and select More, then Open file location.
2) Delete shortcut from Start Menu: Right click on shortcut and select Delete. (See: Adobe reader showing wrong icon in taskbar[Windows 8, 10] - YouTube)
3) Leave open this folder, and change to parent directory (C:\ProgramData\Microsoft\Windows\Start Menu)
4) Open another instance of Windows Explorer.
5) Find folder of Acrobat Reader DC in this new instance. (C:\Program Files (x86)\Adobe\Acrobat Reader DC\Reader\AcroRd32.exe)
6) Drag and drop AcroRd32.exe on Desktop by Right mouse button, then select Create shortcut here. (Or something similar, I am not sure, this is the exact menu, because I have Hungarian Windows version.)
7) Move shortcut created in step 6) from Desktop to Programs element of C:\ProgramData\Microsoft\Windows\Start Menu folder.
It's all, it worked me and I hope it will work you too:
I think I found the solution.
I'm not very sure about whether those steps before the last one were necessary, so you guys can just skip to the last and then back forward to try. Because I cannot make this reappear. 
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2018-9-19
It reappeares! I change the "Open With" of the icon file to Photo UWP App. Just Reset the default apps should work well. The only option of normal icon file is edit action with Panting. So maybe that is why the icon files go abnormal. But why some icons show normal...
First of all, if you can make the thumbnails of icons in the C:\windows\Installer\{AC76BA86-7AD7-1036-7B44-AC0F074E4100} folder (different languages maybe in similar names) show normal, the problem can be solved. dentedharp90041gmpeetbertrand67​​mustafaa47657728KJSTech Thank you guys for the hint! The icons are in good condition and just can't show their thumbnails.
{
1. I delete two types of icon cache databases. "Iconcache.db" (C:\Users\%username%\AppData\Local) & "iconcache_(32/64/...all of the directory).db" (C:\Users\%username%\AppData\Local\Microsoft\Windows\Explorer) AadeshSingh I have tried it though, it didn't work.
2. Right-click on the Computer on the desktop, then select Properties, it will be show your computer's basic information. Then click the last on the left, it may be the Advanced Settings. Then choose the first Set button. Tick "Showing thumbnails, not icons".
[I'm using the non-English version of Windows. You may guess what they exactly means by the meanings.]
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★★★3. Just RESET the default applications in new settings UI.
(Picture 2)
"12.ico" is just a test example, a bit strange though, it just shows.
Before(Picture 3)
Then you may find: The icons come back!
After(Picture 4)
If you succeed, then congratulations! Sorry for the bad language skills, just want to help you guys.
Hello, i think the real problem is that the AppAssoc_de.xml or AppAssoc_en.xml has defined a wrong application for the .ico
so if you have a entry like this
<Identifier=".ico" ProgId="AppX43hnxtbyyps62jhe9sqpdzxn1790zetc" ApplicationName="Fotos" /> in the AppAssoc the AdobeReader Icon in the StartMenu seems to be broken.
The fastet way to fix this is to change the default app for the .ico from Fotos to another app like paint or irfanview!
If you want to broke the AdobeReader Icon in the StartMenu just make .ico to open with Fotos as default Application again.
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I am facing a bit different issue. Icons are displaying normally after I did the Programs > Start Menu shortcut replace thing. All icons in start menu, taskbar etc are showing absolutely fine except for when I search Acrobat DC from the Windows 10 search. I tried many things like rebuilding the icon cache, switching to MS Paint for .ico files, resetting default apps from settings etc. but none of them seem to be fixing this problem. Also, here's the state of files in the below directory before I reset the default apps and after assigning the .ico extension to MS Paint as suggested by many users here. Nothing seems to work though, any help?
C:\Windows\Installer\{AC76BA86-1033-FFFF-7760-0C0F074E4100}
Before:
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After:
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