> Do you have any idea what causes this?

Not yet. Can you install Wireshark and make a snapshot of the wire
protocol communication with the database server in your environment and
at your site (both data source and driver manager)? You can replace user
and password with 'x', but please only the "normal" english letters. I
suspect that it has something to do with the encodings?

Alternatively, you could turn logging into debug mode, only log in this
case will be quite long.

Also, if you can, please try to use fbclient.dll (i.e. type 2 driver) in
data source and driver manager modes (use setType for data source and
change the jdbc url to org:firebirdsql:native in driver manager case).
This might bring some info as well.

Roman