In file https://github.com/svivian/q2a-edit-history/blob/475cb1f9f007a3525c9ac6f0ee79e5cfd6644631/qa-edit-history.php use instead:

content varchar(8000) DEFAULT NULL,

this:

content varchar(".QA_DB_MAX_CONTENT_LENGTH.") DEFAULT NULL,

I just noticed that changes from my 12000 char content in ^posts table were not stored completely.