Hi,
Don't use online websites for sql backups or other methods online with your passwords. Use only phpmyadmin.
You can export sql on phpmyadmin or happens the same?
Make sure you have all plugins disabled.
If you can export your DB on phpmyadmin, save your db tables on your computer.
After that, create a subdomain and install osclass from scratch.
Upload your theme.
Open phpmyadmin again and delete/drop all tables.
All tables was deleted? Good, now import your sql tables backup for your new db name empty.
If no errors during the import, all run well on your subdomain with a fresh install and new db name, now try make a sql backup from osclass admin for see what happens.
This test is for see if with a fresh install osclass with same sql tables, can solve the problem.
Regards