puppet-agent-6.25.0 breaks puppetserver-6.17.0 with multithreading enabled

Puppet Version:  6.25.0
Puppet Server Version: 6.17.0
OS Name/Version: SLES 12SP5

After upgrading puppetserver to 6.17.0 and puppet agent to 6.25.0 (on puppetserver) (running on SLES12 SP5) the agent freeze after some puppet runs complety and shows the following error

Request to https://<puppetserver>:8140/puppet/v3/file_metadatas/plugins?recurse=false&links=manage&checksum_type=md5&source_permissions=ignore&environment=produktion interrupted after 300.002 seconds Wrapped exception: end of file reached

The interesting thing about this error is the timing when the error occurs. Furthermore no errors logged in puppetserver.log, there seems no bottleneck (cpu/memory - each node has 4 cpus / 10G Memory, xms/xmx set to 4G), the error just occurs after some time or load?!

But let me describe observed behavior

1. (re)starting the puppetserver, cpu load rises for some time, but i think this is quite normal 
2. starting 2 puppet agents on other hosts in an endless loop to simulate load ( while :; do puppet agent -t; done)
3. after some more time / puppet runs the error from above accurs

If we downgrade the puppet-agent to 6.24.0 on the puppetserver everything works fine again.

Desired Behavior:

We can update puppet agent and puppetserver to the latest 6.X Version and everything works as expected.

Actual Behavior:

We need to downgrade puppet-agent on the puppetserver hosts to 6.24.0.