Woohoo! Great to hear that 
This sounds like the default client_max_body_size limit on nginx; is there an nginx in front of your mattermost installation? If so, please search its configuration file for client_max_body_size option and increase it accordingly.
You should also see corresponding error messages in the file /var/log/nginx/error.log (if you have that).
Yes, there is nginx, I’ve added client_max_body_size 1024M; after sendfile on;, restarted the service and still same error in nginx log. My admin should take a look at this later.
Thanks for all!
Edit:
Found mattermost.conf inside conf.d Everything is working.
Great, then I’d say we “close” this topic here. Pick whatever message you think works best a a solution so future fellows will have a shortcut to a working slack import and if you experience any other issues, feel free to open a new topic - I’ll be here
I’ve marked the one with the workaround link (with “data” folder missing). Regards!