Docker doesn't start after Nextcloud being installed

Hello, Nextcloud community!

It’s my first topic here, happy such a community actually does exist.
I would really appreciate any advice on the following issue.
Could you assist me with this one, please?

So the thing is that I’ve asked my VPS support rep to install VestaCP and Docker on the fresh server (Ubunru 16.04, Apache).
All was good and Docker was working as it should.

Then I’ve installed Nextcloud (following official manual
Nextcloud is up and running, all is fine and working there (seems - as it should).

But, here is the case - for some unexpected reason Docker just doesn’t give any feel of being alive, actually it doesn’t even start. I need to install Collabora, but seems no chance so far.
I’m calling sudo docker pull collabora/code - no response, the command line is just empty…

Could you maybe direct me - where one should to dig to figure this out?

Looking forward to any hints.