The shutdown situation looked like this:
After deleting SQL Charts files from WordPress via the hosting provider’s interface, the website was open again.
However, the email sent automatically from the WordPress system identifies another plugin as the villain: Table Press. So apparently SQL Charts conflicts with Table Press. After deactivating Table Press, there was no more any problem with installing SQL Charts again.
I attributed the issue to PHP compatibility because I had experienced the same behaviour after updating my PHP version. I could only update after parting with a particular printing plugin. So apparently I was wrong blaming SQL Charts – sorry for that.