When the schema name in database is different from 'dbo', HeidiSQL constructs object names in internal queries like this:
select ... from "DatabaseName".."TableName"
When I manually run same query with a fix - it works fine:
select ... from "DatabaseName".myschema."TableName"
And when database schema name is default (dbo) - Heidi handles it correctly:
select ... from "DatabaseName".dbo."TableName"