Unable to add a subscription to any project inside a group

[intelltec]

I have Mattermost Team Edition Version: 7.5.1 and GitLab Community Edition [15.5.4] self-hosted both and the latest for today. Got several projects at GitLab combined into a group. Using genuine guide from Readme.md. Everything is fine up to Step 4 and there: If I'm using /gitlab subscriptions add thegroup issues,merges,pushes,pipeline,tag the group is connected. I don't need to connect the group, because I cannot assign webhook to it since I'm non-premium GitLab user. So, if I'm trying to connect any of my projects by /gitlab subscriptions add thegroup/theproject issues,merges,pushes,pipeline,tag - I'm getting an error 'Resource with such path is not found.' from the GitLab bot. I tried to start over with different Gitlab accounts, projects etc. - every time I cannot connect a project which is inside a group.

So the steps to reproduce will be:

1. Install Mattermost Team Edition Version: 7.5.1 and GitLab Community Edition [15.5.4]
2. Create three projects in Gitlab project{1,2,3}, and group them into 'mygroup'
3. Install mattermost-plugin-gitlab
4. Configure it precisely following the instruction up to step 4
5. Execute /gitlab subscriptions add mygroup/project1 issues,merges,pushes,pipeline,tag or /gitlab subscriptions add mygroup/project1
6. The error - is a denial from GitLab bot "Resource with such path is not found."