Connecting to a FB3-Database via localhost works only as long as a network connection is available.
A disconnected machine is unable to connect to databases via isql or flamerobin.
Initial Ok-state:

SQL> connect localhost:k4.fdb user sysdba password masterkey;
Database: localhost:k4.fdb, User: SYSDBA
SQL> quit;

When the machine is disconnected from the network (network-cable plugged, Wifi off), the connection is no longer possible.

SQL> connect localhost:k4.fdb user sysdba password masterkey;
Statement failed, SQLSTATE = 08006
Unable to complete network request to host "localhost".
-Failed to locate host machine.
-The specified name was not found in the hosts file or Domain Name Services.

Although connection to 127 is still possible.
SQL> connect 127.0.0.1:k4.fdb user sysdba password masterkey;
Database: 127.0.0.1:k4.fdb, User: SYSDBA

Connecting via file access through the client library works too.

So:
Somehow FB3 seems to require a working network device to be able to connect to a database via localhost.
FB2.5 did not have this issue.
Maybe this is related to the new Providers mechanism in firebird.conf? (#Providers = Remote,Engine12,Loopback)
The FB3-Server-process also seems to have issues terminating when it was started with Providers are selected explicitly.
Stating 127.0.0.1 as localhost in the hosts-file under C:\Windows\System32\drivers\etc does not seem to help.