Thank you for your instructions, AileneA. I remain frustrated; here what I have tried:

At first, staying with the Chrome browser:

** Decided to accept the default dates when I ask for a statement of account, which means a start date of 2020/6/21. But first, I ran the Customer (=Member) Balance Summary for 2020/6/20.
** Selected Joe Salz as the member (=customer) for whom to generate a statement. Orange error box appeared as before.
** Cleared all cache and cookies from Chrome browser. Signed out of QBO; signed back in.
** Ran Member Balance Summary again for 2020/6/20.
** Tried to generate statement for Joe Salz again. Orange error box appeared again.
Switched to Firefox browser:

** Cleared all cookies and cache.
** Signed in to QBO. Ran Member Balance Summary for 2020/6/20.
** Selected Joe Salz as the member. Tried to generate statement. Orange error box appeared as before.
** Selected Dan Alfus as the member. Tried to generate statement; worked just fine.
** Went back to Joe Salz --> statement --> orange error box.
Switched to Microsoft Edge browser (the new that comes with Win10 2004):

• Cleared cache, history and cookies
• Signed in to QBO. Ran Member Balance Summary for 2020/6/20
• Tried to generate Statement for Dan Alfus (positive balance) --> worked ok
• Tried to generate Statement for Joe Salz (0 balance) --> orange error box
• Tried to generate Statement for John Frischkorn (positive balance) --> orange error box

I hope you can fix this soon!