> and even connect from <firebird>/bin/isql using the same string just
> fine.

What command so you execute?

CONNECT 'localhost/3050:/opt/firebird/examples/employee.fdb' ...
or
CONNECT '/opt/firebird/examples/employee.fdb' ...

If latter, ensure that your server is running, for example by trying the
first connection string.

Also check if the database is writable by the firebird group, not only by
the firebird user.

Roman