I upgraded production database, let it working for some weeks, then i realized that due some limitations (upadte of system tables) i am not able to use it with 3.0, so i tried to downgrade it using this way: 

- i got zip of fb 2.5 and unpacked on the fb 3.0 server

- enabled Legacy_Auth for the 3.0 instance

- used gbak -B -V from 2.5 folder 

- restored successfully on fb 2.5 server 

- gbak backup and restore on the pure 2.5 instance 

The problem: only sysdba and the user who restored the database can access it. On the rest, when try to read someting it fails with no permission for read/select access to DATABASE

c:\Program Files\Firebird\Firebird_2_5\bin>isql -user SOM -password secret 
Use CONNECT or CREATE DATABASE to specify a database
SQL> connect "c:\data\data.fdb";
Database:  "c:\data\data.fdb", User: SOM
SQL> select * from exampletable ;
Statement failed, SQLSTATE = 28000
no permission for read/select access to DATABASE
Command error: select * from exampletable

Same question, but with other words (as i was unsure if it is about the downgrade) published also in SO https://stackoverflow.com/questions/58920660/after-downgrading-database-fb3-to-fb2-5-no-permission-for-read-select-access-t#58920660 

FB versions are the recently ones: 3.0.4 + 2.5.9