Hi,
the extension can't sends messages anymore.
Try to update the extension to the latest version, clean LHC cache but same issue.

From Mattermost log I have only this:

{"[censored CODE]","method":"POST","err_where":"Channel.IsValid","http_code":400,"err_details":"id=[censored ID]"}

Any suggestion?
Current Mattermost version: 8.1.2

Thank you.