This is weird.
So I tried a preview of a Word document, located on D-partition. Opus shows "This Microsoft Office document could not be displayed."
Office 365 is installed, so maybe it had something to do with the handler or so, whatever? Checked Internet, but wasn't sure where to look for what and what to do. Gave up.
Copied the file in question to another partition (Q), hit F9 preview and the contents was shown without problems.
Switching tabs: on one tab no preview, the other tab, different partition preview is fine.
Again copied to another partition (E), preview fine.
Opus File>Exit + Relaunch no difference.
FWIW Note that Windows Explorer does not show a preview either, no matter .doc(x) .xls(x) .csv. .txt - it only shows a preview of images. This has always been the case. Didn't care, never used Explorer.