the table column on mauticemail_stats is
copy_id varchar(32) DEFAULT NULL,
and on mauticemail_copies it’s defined as
id varchar(32) COLLATE utf8mb4_unicode_ci NOT NULL,
so you need to make sure both tables are encoded with the same collation.
You need to convert all the tables into the same (preferability utf8mb4_unicode_ci) on all tables, there’s some other posts on this forum and github related to this issue. One issue you may face after trying to convert the columns to utf8mb4_unicode_ci, is that the index is bigger than what’s supported on mysql/mariadb, so you need to reduce the column from 255 to 191 chars