Import Slack into RocketChat

Hello,

I tried to import my Slack data to RocketChat.
I followed this guidline
https://docs.rocket.chat/guides/administration/admin-panel/import/slack11
I can select the channels I like to import and click the Import button.
Then I only get an “Imported_Operation_Failed” error and nothing else happened.

What could be the problem?

The same error occours when I only choose one channel.
Thank a lot