Hi,
I finally found out why my "Get options from MYSQL query" setting couldn't be saved.
The server had a WAF setting (Web Application Firewall) that detects access to SQL statements. I turned it off, and I was able to save.
There was also a WAF setting that detects 'kill, ftp, mail, ping, ls' commands that will case 501 error on saving record for Newsletter Builder.
Sorry if I took your time.
In case someone has the same problem, you might want to check the server settings.
Best,
Hiroko