docx files created under 5.1 or MS Word won't open correctly under 6.1

 I created a long document under version 5.1 saved in docx format then upgraded to version 6.1. When I tried to open the docx file under version 6.1 all it showed were blank pages or short lines of corrupted text (overprinted). Also 6.1 won't open my older docx files created using MS Word of which I have thousands. 
I recovered my document, which I had printed copy out under ver. 5.1 using Adobe pdf converter. Adobe generated docx files which opened in version 6.1, but understandably, with imperfect formatting. I've used ver 5.1 for several months and other than getting used to some difference between it and Word it's a great program. That said, this current problem is very distressing and I hope there is an easy solution.
Windows 10
  1. Please attach a sample file created with 5.1 or MS Word, which used to open fine in 5.1, but fail with 6.1.
  2. If that turns out to be a real bug, you would need to file a bug report to bug tracker, so that developers be notified and could act on it.
  3. In the meanwhile, (if that’s a bug, and not some glitch fixed by profile reset), you can downgrade to any version that works for you, using our download archive.

This is SMaitland
I’ve learned a lot since I posted my concern last night. Unfortunately, the problem is more severe and pervasive than I first suspected:
First I verified that my apparently corrupted docx files in LibreOffice 6.1 still opened correctly in MS Word, so the
file was still OK—Thank God!
Next I downloaded an older version of LibreOffice. I couldn’t remember exactly which version of 5 I had, so I tried and guess what? It also produced unreadable files. So with nothing but time to kill, I removed it and
downloaded ver. and—voila! The docx files open like a champ! (that’s technical speak for Thank God).
Yup it looks like a real honest to God docx file reading bug. I just checked that docx files I recently created
under ver 6.1 can be successfully opened by without issue. Somewhere after version a bug
slipped into the code.

I repeat again: you need to provide something to developers to allow them fixing something you call a bug. If there was a widespread problem, it would be already caught and fixed; so it must be something specific to your documents that reveals the bug in LO. Please file a bug report with a sample file and specific description what looks broken, and where it looks normal. Without that, nobody would be able to fix what only you see.