I'm also having this problem, using a Google BigQuery connection.
We have many other apps using the same connection, many in GUI mode. Everything still works, but I can't set up a new GUI mode query because just today I am seeing "no data" in the select a table dropdown.
I had a look at one of our other apps (using the same account, same BigQuery connection) which use GUI mode queries, and the correct table is still displayed, but clicking on the dropdown there also shows "no data".
Very frustrating, please be aware this is still an issue, and not just for PlanetScale