What you say is exactly what's happening.
We received the question about the error in what we think is an OJS 2.3.7 version as that's the link we received from the user.
So, we downloaded the master.tar.gz from this link
We don't remember exactly how we got there, except that we found the tar.gz after accessing the DOI PUB ID plugin
page we followed from a link somewhere, after searching for DOI plugin, or something like that.
Since this tar.gz has a README file saying it's OJS 2.3.4, we assumed the plugins would work, and copied the plugin folder and ran the tools/upgrade.php upgrade to install.
When we run it, in a 2.3.8, it shows the aforementioned error.
Basically, this problem is solved.
Now, to the next question:
This mEDRA is just for Europe, right? Or can it be used to register DOIs at CrossRef?
Ramón Martins Sodoma da Fonseca
Analista em C&T
Coordenação de Tecnologias de Informação
Instituto Brasileiro de Informação em Ciência e Tecnologia - IBICT
Ministério da Ciência e Tecnologia - MCT