Snap refresh "core" is not possible

Hello,

i’m using ubuntu 18.04.04 LTS
We use SNAP for Rocketchat. If i try to run sudo snap refresh i got this error:

Error: cannot refresh snap-declaration for “core”: Get https://api.snapcraft.io/api/v1/snaps/assertions/snap-declaration/16/99T7MUlRhtI3U0Fgl5mXXESAiSwt776?max-format=3:
Forbidden

My Versions are:
core: 16-2.43.3
RocketChat 2.4.11

How can i solve this problem?
Thanks and best regards

EDIT.: Problem solved. We had in the past a proxy running. We moved the Server to a VLAN where the proxy is not necessary.
The proxy was still configured in the file /etc/systemd/system/snapd.service.d/snap_proxy.conf
I commented out the proxy entries and the refresh command is working fine.