I've found a solution without using the root user.
I'm using XAMPP & phpMyAdmin to handle MySQL DBs.
Even with granted admin rights for a user, the project transfer will fail - late versions of EA V15 and also V16 has this issue, when transfering from .QEA to DBM. (EAPX->QEA as intermediate transfer is definitely recommended, I've had issues with transfer attempts without .QEA as source format in the past)
So, the patch:
1. Go to phpMyAdmin on the server
http://localhost/phpmyadmin/index.php?route=/2. Select the created empty DB (after applying the current schema (EASchema_1558_MySQL.sql) and initializing the tables (EABase_1558_MySQL.sql))
3. Select User Rights
4. Select the User you intend to use with user/pass in the transfer (QEA->DBM) session
The "Change Rights" site has 4 tabs: Global, Database, Change Password, Login
Choose Database.
5. On this tab, you find a dropdown list where additional rights can be given for this user. (although one would assume that it is sufficient that is already configured, but its not...)
From the dropdown list, select the prepared empty database. It will add a row in the Database-specific rights section in this tab.
"ALL PRIVILEGES" are needed for this entry.
6. At this point, you shall switch to EA and attempt the project transfer with the user account. It shall succeed.
7. If you check again the user permissions in phpMyAdmin, the specific user received a Database-specific placeholder in the user overview. This can be removed after the successful model transfer, it was only necessary to unblock the transfer mechanism. (All other operations worked for me after removing the placeholder.)