Hello,

It is not possible to add a comment to the article. No errors, message show that comment is addet successfully. But comment do not appears in frontend and backend? Commenting is enabled in configuration. What could be wrong?

@Mindaugas
Enable error login and db logs and look for errors or invalid SQL statements.

No erros in SQL. Only template error:
[11-Nov-2021 16:24:48 Europe/Vilnius] PHP Warning:  Cannot modify header information - headers already sent by (output started at /public_html/oc-content/themes/alpha/head-color.php:29) in /public_html/oc-includes/osclass/core/Cookie.php on line 106

In which table the comments are stored?

@Mindaugas
Such error is never theme related, in that file on line 29 is basically nothing, just output of variable.
Comments are stored in t_item_comments

I found that blog comments stored not in 't_item_comments', but in 't_blog_comment'. That may be the problem that it doesn't display a comment in blog frontend and backend? After set parameter 'b_enabled' to 1 in table 't_blog_comment' for comment, its start showing in frontend.

And another bug in blog backend comments filter: if filter set to 'Enabled&Disabled' (and all comments on table 't_blog_comment' parameter 'b_enabled' is status 0) => no comments show, if set to 'Enabled' or 'Disabled' => it start show comments by status, if one of comment parameter 'b_enabled' is set to 1 and filter in backend set to 'Enabled&Disabled' is show only enabled comments.

sorry for my english

@Mindaugas
They are disabled but still should be visible in plugin backoffice.
Please create support ticket with all the details and add backoffice & ftp login.

How and where i can create support ticket?

Here it is:
https://osclasspoint.com/support/tickets