Cannot Preview Network Files Within File Explorer

I am also facing this making my work much slower - everytime I have to open the document to see its contents. 

Until recenty, every file preview was visible in the preview pane. Since the last few days, preview of some files are visible; for some, I get an error message -  'The file you are attmpting to preview could harm your computer. If you trust the file and the source you received it from, open it to view the contents'. But, previews of the ones with error message were visible a coupl eof weeks back and now they have become harmful! 

Can someone please help solve this issue with Windows Explorer?