Answer: Are you ready for the simplest explanation ever? 🙂
The reason is that -most probably- (hey, you never know) you are using a SQL Server login that has conflicting permissions.
More specifically: The login you with which you are connected to the SQL Server instance, has the db_denydatareader database role assigned. This means, that whatever else database role the login has assigned to it, even db_owner, the db_denydatareader will be in conflict thus not allowing to access certain resources of the database.
How to fix that: If it is OK with your security requirements, remove the db_denydatareader role from the login. After that, you will be able to access the mapped database (if course the rest of the required permissions are in place).
Similarly, have in mind that you get the same behavior if you have write access to the database but also have the db_denydatawriter database role assigned to the login.
In general, be careful with the database roles you assign to any login and avoid giving conflicting accesses.
Recommended eBooks on SQL Server: