Flippity Add-on for Google Sheets. The trouble was that after years of successful use, people were getting a warning message from Google that said the Flippity Add-on was unverified and not recommended for use. I reached out to the developers of Flippity back in May and was told that they were aware of the issue and were working to resolve it. (By the way, Flippity wasn't the only Add-on that had this trouble in the spring).
This evening I checked on the Flippity Add-on for Google Sheets and it seems to be working again without any problems. I was able to successfully install it and use it in three different Google Accounts. So if you were having problems with the Flippity Add-on for Google Sheets in May or June, give it a try tonight and see if it works for you.