Problem "Add Series" function - "That question isn't compatible"

With the recent update, the problem with the "Add series" function has been fixed, and I could carry out connections without any problem.

However, since a few days, I have been getting the error message "That question isn't compatible" when connecting questions from different tables in a dashboard. I repeated this process with other questions I had been able to carry out before and received the same error message. If the questions are based on the same table, there is no problem, and Metabase lets me do it. But when I try to mix questions from different tables, I get this error message.

What could it be?