Unexpected error after 4 hours of VM backup

[mummonth]

Hello, Community!
My case number is 00655812.
I am trying to backup VM from our VMware ESX.
It seems to be trying something for a long time, but shows 0% progress all the time.
And after working 3 hours 53 minutes job failed with unexpected error.
I analysed logs and found that there is the same loop repeating all the time, trying to find free proxy and failing:

And this process repeats every minute trying to find free proxy and failing in the end.
What could be the issue?

[foggy]

Artem, how many proxy servers do you have and what are the max concurrent tasks number for them? Do you have other active tasks running at the same time? Could you please also describe your setup in a bit more detail? Thanks.