we have some issues trying to import a large Slack backup (1GB compressed 11GB uncompressed) to mattermost, we are using mm 5.18.0 and using the CLI to import the original zip file from the corporate slack export (with private channels and direct messages).
when trying to import the complete export, the server show a large number of import errors regarding file ids not being found besides bot accounts not being created (bots are not important). So as a debug, we created a single day export of the last available day on slack (4.5 MB compressed), when trying to import this file, we always end up with a server error
mattermost panic: runtime error: invalid memory address or nil pointer dereference after a few channel import errors regarding channel name length.
at the moment we are trying to import a new fresh export to discard export file corruption (this process has taken around 7 hours to complete at it seams that the import file process before committing changes is single thread)
FYI we are importing using the command
sudo -u mattermost ./bin/mattermost import slack <team name> <file name>