@p44 The MySQL database in Cloudron is shared by all the apps. In MySQL 5.7.5+, they changed the behavior to have ONLY_FULL_GROUP_BY to be the default to be more compliant. Disabling this at the server level will cause issues for other apps.
The correct way to fix this is to have the app disable this mode at a session level. Like https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html . Essentially, the app has to call "SET GLOBAL sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));" after creating a connection.
So, I guess the answer is:Either sendy has to provide a mechanism to do the above. Alternately, you cannot run sendy in Cloudron. You have to create a dedicated server for this and setup MySQL accordingly.