I can't open docx files in Word

With LibreOffice installed, but not running, I can’t open docx files in Word. They will open in LibreOffice, but LO files saved as docx won’t open in Word. Doc files open fine in Word. Any ideas?

LibreOffice of which version? Word of which version? Windows (?) of which version? “I can’t open docx files in Word” - how are you trying to do that? “LO files saved as docx won’t open in Word” - why? because they are opened in LO? or because Word emits an error (which)? So: is your problem the association of DOCX with LO, or broken DOCX files produced by LO?

Sorry, very fair questions.
LibreOffice Version: 6.3.4.2 (x64)
Build ID: 60da17e045e08f1793c57c00ba83cdfce946d0aa
MS Office Home and Business 2016
MS Word Version 2001 (Build 12340.20184)
Windows 10 Home 10.0.17763 Build 17763
I’ve tried opening docx files from Windows Explorer and from Word itself - in neither case will a docx open. There is no error message, Word attempts to open the file but nothing happens.
It doesn’t matter if the docx files were created in Word or in LO. It also doesn’t matter if LO is running or not, the problem still occurs.
Docx files created in LO or Word open in LO.
If I uninstall LO the problem disappears.
Doc files (from earlier versions of Word under Windows 97) open correctly with LO installed or not.
So, my problem is neither the association of docx with LO or broken docx files produced by LO. The problem seems to be that installing LO prevents docx files from opening in Word.
By the by, xlsx files open perfectly in Excel.

I’ve tried opening docx files … from Word itself - in neither case will a docx open. There is no error message, Word attempts to open the file but nothing happens

Absolutely strange… there’s no way LO could break something inside Word - but “If I uninstall LO the problem disappears” - unbelievable… (FTR: I have both LO 6.4.1.1 installed normally (and all older versions in parallel) and MS Office 2016 installed on my box, without any problems…)