I am experiencing a steady drop in organic traffic & impressions since May for one of the clients.

Posted by: @aldesylcen

The drop is not limited to a particular page or keywords - it is across the board.

I also found that the Full version of Google Cache is not loading. It is just a blank page. However, the text version is loading properly. In GSC, the URLs are indexed and there are no errors.

What can be the reason for the full version not getting loaded? Should I be concerned about this? There are HTML validation errors (including a fatal error) but they are present on my other website as well without any impact.

Would really appreciate any thoughts/insights on this.

Sounds like you either A) Don't have relevant content or B) you were hit with some kind of penalty or something on the site isn't right. C) you probably should update the content and provide real value to your readers and get some high-quality backlinks.  D) Check the entire website for pages that might be poor quality it almost seems like Google is evaluating an entire website vs 1 or so pages now days.