[strongSwan] allocating SPI failed: Unknown error (524)?

[hb]

Another (stupid) question, after authenticating to the server I get

allocating SPI failed: Unknown error (524)
unable to get SPI
allocating SPI from kernel failed

# ipsec version
Linux strongSwan U5.5.1/K4.7.9-64
Institute for Internet Technologies and Applications
University of Applied Sciences Rapperswil, Switzerland
See 'ipsec --copyright' for copyright information.
#

The Linux kernel is
# uname -rmv
4.7.9-64 #19 SMP PREEMPT Sat Oct 22 02:39:01 CEST 2016 x86_64
#

524 looks like errno ENOTSUPP, but what do I know. Not supported smells
like not configured during the kernel build. What do I have to check
and/or to change?

Thanks,
Hartmut