Error - T.38 gateway already running

I tried to send a fax from an ATA which is set up as a T.38 gateway extension. The fax dials and answers, but fails to transmit. This error is displayed in the log:

[2016-04-21 13:28:36] WARNING[10063][C-000141fb] res_fax.c: Attempt to attach a T.38 gateway on channel (SIP/11205-0001ed50) with gateway already running.

Google search showed nothing. Any ideas?