ERROR: too many dynamic shared memory segments

Hi,
I started seeing those errors on Postgresql 10.1 running on Ubuntu 16.04.3
(64GB/8CPU) server.
The DB enters recovery mode after that.

2017-11-23 07:20:39 CET::@:[24823]: ERROR: could not attach to dynamic
shared area
2017-11-23 07:20:39 CET::@:[24822]: ERROR: could not map dynamic shared
memory segment
2017-11-23 07:20:40 CET:192.168.xx,xx(33974):u(at)db:[24209]: ERROR: too
many
dynamic shared memory segments

The errors happen when the parallel execution is enabled and multiple
queries are executed simultaneously.
If I set the max_parallel_workers_per_gather = 0 the error doesn't occur.
The queries are rather big, each executes over 10sec.
I see the error when 4 or more queries are started at the same time.

my postgresql.conf:

max_connections = 100
shared_buffers = 12GB
effective_cache_size = 51GB
work_mem = 83886kB
maintenance_work_mem = 2GB
max_wal_size = 8GB
min_wal_size = 4GB
checkpoint_completion_target = 0.9
wal_buffers = 32MB
default_statistics_target = 1000
max_locks_per_transaction = 128
#max_parallel_workers_per_gather = 0

PS. I've posted this question on pgsql-admin distro but didn't get any
feedback.

--
regards,
Jakub Glapa