The customer was experiencing off SQL*net problems, so I did a client side trace. The tracing worked just fine, except the trace files were written to the wrong location. Instead to a subdirectory of my home, they went to the ADR (Automatic Diagnostic Repository) as usual.
The solution:
trace_level_client=16
trace_file_client=client
TRACE_TIMESTAMP_CLIENT = ON
TRACE_UNIQUE_CLIENT = ON
trace_directory_client=/home/lhaake/net
DIAG_ADR_ENABLED= OFF
The last two parameters are crucial.
trace_directory_client: this is where the logfiles go.
DIAG_ADR_ENABLED: set this to send the log files to the above folder, instead to the ADR repository