I added some OPML and Scrivener templates and woof my notes preview area has all turned grey!
Restarting has got it back, but sometimes seeing grey suddenly
Check the screenshot below.
Not an issue users can help with. Best take this to support. 
FWIW, in c.18 years of use, that’s a new one for me. Then again, as with any software, once you’ve found a glitch it keeps on giving. Often the hard part is figuring out what we did to cause the effect, which might be a sequence we individually do a lot but it transpires no others users do, thus the novel discovery.
Perhaps you could post the offending document?
Will email you tomorrow morning. (India)
Emailed to [tinderbox@eastgate.com]
Works fine here! Working to resolve the issue.
Cool, will let you know if it bothers me again. For time being deleting the OPML template and restarting has resolved issues at my end.