Can't create more than 494 threads

[info at dubistmeinheld.de]

Hi,

I'm using Varnish since a couple of years and I'm very satisfied. Thank
you for the work!

Recently I setup a new server with Varnish 5.0. For some reasons,
threads_created is stuck at 494 and threads_failed increase every
second, even with no load (-s malloc,2G -p thread_pools=2 -p
thread_pool_min=250 -p thread_pool_max=2000 -p thread_pool_fail_delay=2).

MAIN.threads                    494          .   Total number of threads
MAIN.threads_limited              0         0.00 Threads hit max
MAIN.threads_created            494         0.01 Threads created
MAIN.threads_destroyed            0         0.00 Threads destroyed
MAIN.threads_failed             356         0.01 Thread creation failed

I tried now for a couple of days to fix this issue and I'm looking in
the area of a kernel param. But I'm completely stuck on what's causing it.

I would appreciate any help and pointing me into the right direction.

Best regards,
Jens