I shared the errors already so many times (check this mailinling for "solr" in teh title)

Contrary to what you say, with SOlr 7.5 and Dovecot git,  I had to remove the "managed-schema" to make solr respond a bit properly. It relies on schema.xml

In order to create the instance, no, it copies  the default config in the dovecot instance.

I'm not a Solr expert by any means but I believe you are incorrect.

As of Solr 5.x the managed-schema file is the primary method for configuration.  The method I detailed previously for setting up a config helps automate creating new Solr instances - but as I stated you can either setup a Solr template and then create the instance from that or create an instance using the default template and then adjust it.

The part that you *must* do after creating from the default template is stop the server, delete the entire "<prefix>/solr/dovecot/data" folder, then install the correct managed-schema file, then restart the server.  The server will not function with mismatched schema/data.

If you'll try that - explicitly "rm -rf <prefix>/solr/dovecot/data", copy the managed-schema file into the conf folder, and restart - things will either work or there's something else that needs correction.