ERF12-5356 [ProxyAPI::ProxyException] with Foreman 1.8.1

I upgraded recently from Foreman 1.8.0 to 1.8.1 and from Puppet 3.7 to
Puppet 3.8.1 (on Ubuntu 14.04). When I restart the server, I get the
following error when trying to display certificates in the webui:

ERF12-5356 [ProxyAPI::ProxyException]: Impossible d'obtenir les certificats
PuppetCA ([RestClient::NotAcceptable]: 406 Not Acceptable) pour le proxy
https://puppet.cptaq.local:8443/puppet/ca

Here's the log of foreman-proxy:

I, [2015-05-29T13:00:28.212261 #1054] INFO – : Running scan_directory on
test: /usr/share/puppet/modules
10.17.80.5 - - [29/May/2015 13:00:28] "GET
/puppet/environments/test/classes HTTP/1.1" 200 92433 4.7505
D, [2015-05-29T13:04:14.858340 #1054] DEBUG – : verifying remote client
10.17.80.5 against trusted_hosts ["puppet.cptaq.local"]
D, [2015-05-29T13:04:14.859141 #1054] DEBUG – : Found puppetca at
/usr/bin/puppet
D, [2015-05-29T13:04:14.859232 #1054] DEBUG – : Found sudo at /usr/bin/sudo
D, [2015-05-29T13:04:14.859277 #1054] DEBUG – : Executing /usr/bin/sudo -S
/usr/bin/puppet cert --ssldir /var/lib/puppet/ssl --list --all
W, [2015-05-29T13:04:15.276960 #1054] WARN – : Failed to run puppetca:
E, [2015-05-29T13:04:15.278112 #1054] ERROR – : Failed to list
certificates: Execution of puppetca failed, check log files
10.17.80.5 - - [29/May/2015 13:04:15] "GET /puppet/ca HTTP/1.1" 406 74
0.4209
D, [2015-05-29T13:04:15.472967 #1054] DEBUG – : verifying remote client
10.17.80.5 against trusted_hosts ["puppet.cptaq.local"]
D, [2015-05-29T13:04:15.473276 #1054] DEBUG – : Found puppetca at
/usr/bin/puppet
D, [2015-05-29T13:04:15.473382 #1054] DEBUG – : Found sudo at /usr/bin/sudo
D, [2015-05-29T13:04:15.473421 #1054] DEBUG – : Executing /usr/bin/sudo -S
/usr/bin/puppet cert --ssldir /var/lib/puppet/ssl --list --all
W, [2015-05-29T13:04:15.837424 #1054] WARN – : Failed to run puppetca:
E, [2015-05-29T13:04:15.838496 #1054] ERROR – : Failed to list
certificates: Execution of puppetca failed, check log files

If I simply restart the foreman-proxy service, everything works fine.
Here's the log:

10.17.80.5 - - [29/May/2015 13:04:15] "GET /puppet/ca HTTP/1.1" 406 74
0.3663
W, [2015-05-29T13:06:11.993819 #8093] WARN – : Couldn't find settings
file /etc/foreman-proxy/settings.d/foreman_proxy.yml. Using default
settings.
I, [2015-05-29T13:06:11.993979 #8093] INFO – : 'foreman_proxy' settings
were initialized with default values: :enabled: true
I, [2015-05-29T13:06:11.997012 #8093] INFO – : 'facts' module is disabled.
I, [2015-05-29T13:06:11.997477 #8093] INFO – : 'dns' module is disabled.
I, [2015-05-29T13:06:11.997749 #8093] INFO – : 'templates' module is
disabled.
I, [2015-05-29T13:06:12.001059 #8093] INFO – : 'dhcp' module is disabled.
I, [2015-05-29T13:06:12.328149 #8093] INFO – : 'puppet' settings were
initialized with default values: :cache_location:
/usr/share/foreman-proxy/cache, :puppet_provider: puppetrun, :puppetdir:
/etc/puppet, :salt_puppetrun_cmd: puppet.run, :use_cache: true
I, [2015-05-29T13:06:12.330752 #8093] INFO – : 'bmc' module is disabled.
I, [2015-05-29T13:06:12.331117 #8093] INFO – : 'realm' module is disabled.
D, [2015-05-29T13:06:16.287769 #8098] DEBUG – : verifying remote client
10.17.80.5 against trusted_hosts ["puppet.cptaq.local"]
D, [2015-05-29T13:06:16.290139 #8098] DEBUG – : Found puppetca at
/usr/bin/puppet
D, [2015-05-29T13:06:16.290256 #8098] DEBUG – : Found sudo at /usr/bin/sudo
D, [2015-05-29T13:06:16.290314 #8098] DEBUG – : Executing /usr/bin/sudo -S
/usr/bin/puppet cert --ssldir /var/lib/puppet/ssl --list --all
10.17.80.5 - - [29/May/2015 13:06:17] "GET /puppet/ca HTTP/1.1" 200 3908
1.0886

I've checked sudoers file and it looks fine:

root@puppet:/etc/puppet/environments/production/modules# cat
/etc/sudoers.d/foreman-proxy
foreman-proxy ALL = (root) NOPASSWD : /usr/bin/puppet cert *
foreman-proxy ALL = (root) NOPASSWD : /usr/bin/puppet kick *
Defaults:foreman-proxy !requiretty

Any ideas of what is different in the boot process for the foreman-proxy
service?